There are some possibilities to consider.
This could be a different side of the cube (unlikely)
I'm fairly confident we do not see all sides of the cube, but for instance on page 138, we see a different side, and it has some different damage.So could we just be looking at a side of the cube we haven't seen?
Probably not, actually. I do not think this possibility is very likely, because we see Chito and Yuuri's stuff is still there. They could have moved it, but continuing from the preceding scene, it is very likely the intent is to convey a continuity of the same space. The staircase is also shown, the positioning of which should help us determine whether it is the same face of the cube.
Time could have passed, but the footprints could still remain because it is not snowing
Consider for a moment why footprints in the snow disappear. More snow falls and covers them up, or the wind blows more snow onto them. Or the sun melts the snow. If none of these are happening, then the footprints could remain there indefinitely. Initially, I thought this theory was sunk as soon as I saw the cover and with Chito and Yuuri covered in snow, but this is clearly the result of their snowball fight in the last chapter, as it is only after that point that they are shown to have snow on them. During the last chapter, I had some difficulty figuring out if the pictures were showing snow or stars, but on pages 136-137, where the sun is rising and Yuuri and Chito are looking out at what's below them, we can see it is not snowing, so it is indeed stars. There are wispy clouds, but no snow. Also, though the picture is kind of difficult to decipher, Yuuri and Chito say they're going to the edge, so I think many of the clouds are actually below them, and they're looking at the layers beneath them.
So, it is not snowing on the top layer at any point during which we observe. Additionally, even though the sun comes up, it does not melt the snow. What possibilities can we infer from these things? Let us remember for a moment how high up the two of them are. Any atmosphere or weather up here might be artificial. And given the decaying state of the whole world, it wouldn't be strange if the weather was broken, and thus it isn't going to snow anymore. Would it be completely unreasonable to think that footprints would last for a very long time?
Now, one problem I have with this theory is that some of the footprints actually do seem to have disappeared. There should be some that go all the way back to the stairs, since that was their arrival point. To defend the theory, I would suggest wind and the sun may have removed some footprints, but that the cube is shielding the remaining footprints we see.
On the other hand, the cover picture of volume 6 does not show footprints to the staircase, even though it appears to be just after their snowball fight, so perhaps we don't need to go to such lengths to explain why those ones disappeared. The two may just not have left any noticeable footprints in the snow until they got closer to the cube.
Some amount of time passed, but it wasn't as long as we expect
We suspect a very long amount of time has passed because the degradation of the cube has increased by a large amount. But why? All of that could have happened at once, as it simply reached its breaking point and just fell away. Chito and Yuuri may even have purposely chipped away at it or used their last explosive on it (it is quite conspicuous that Chito mention their last explosive, but we don't see them use it). On the page after what you've shown in your question, we see a close up of the damaged spot, and there are some symbols on the black part. Maybe they wanted to see that. Or if it is a hole as everyone seems to believe, maybe they wanted to go inside to have a little bit of shelter from the cold.
We cannot determine how much time has actually passed just from the damage to the cube, and we cannot conclude whether it is unreasonable that the footprints would still be there (especially when you take the previous theory into consideration).
As far as Chito and Yuuri causing the damage, there is a fan theory that the cube transported them to the world of Shimeji Simulation. Not having read that manga, I can't speak much to it, but here's what tvtropes.org says:
Call-Back: Shimeji Simulation, being a work created by Tsukumizu, features a few references from Girls' Last Tour:
The first page of Chapter 1 features what looked like a wheat farm, and with Shijima's neighbours, who turn out to be identical to Chito and Yuuri, sitting at a bench on the top-right corner. This was almost identical to the final page of Girls' Last Tour's Volume 6, where Chito and Yuuri were standing at what looked like a wheat farm, as well.
Both Shijima and Majime later encounter a mysterious black cube on Chapter 8, when Majime comments about it having a modern feel. This is a reference to the almost identical black cube that Chito and Yuuri encountered after reaching the highest layer of the megacity in Girls' Last Tour's Grand Finale.
In Chapter 13, Shijima's neighbours look like Chito and Yuuri, as seen in Chapter 1's first page. There were many speculations and fan theories in regards to both of them being alive in Girls' Last Tour after their Big Sleep, and somehow got transported into this world through the mysterious black cube.
Chapter 42 has both Shijima and Majime wearing what appeared to be suspiciously identical helmets that Chito and Yuuri wore.
Though none of this is very relevant here, it is interesting that the cube is also in that story. Thus, if you want to see more of this cube, you can try reading Shimeji Simulation, and see if that helps with figuring out its secrets. Regardless of Shimeji Simulation, it is possible Chito and Yuuri used their explosive to damage the cube for some reason.