In Boruto, we figure out that losing passion towards art will make you lose the ability to perform Ninja Art: Super Beast Scroll.However, Sai had completely suppressed all emotions when he came out of the Foundation.
So, how was he able to use this jutsu? I think passion is also an emotion.
In the anime, for the drawings to retain stability upon emerging, the user's emotional state must have a true passion for them (art). Otherwise, the drawings will immediately splatter.
It is true that Sai didn't had emotions or FWIW, he suppressed them but he didn't lose his passion for art. In fact, it was the one thing he held onto from his early childhood with his brother until the present. His passion for art is what gives him the ability to use the Super Beast Scroll jutsu.