You can either answer based on the series or guess based on knowledge about general Japanese culture / language.
In S01E01: Yuni addresses Mineaki as Mineaki-san.
In S01E05: Yuni addresses Mineaki as Mine-kun after Yume and Mizuto give Yuni a mother's day present.
The only relevant difference I see here is that Yuni knows Yume and Mizuto are around for S01E05. (In S01E01, Yuni doesn't necessarily know that Yume and Mizuto are on the 1st floor of the house.)
Maybe related: Why does Yor call Loid as Loid-san when with others?