Many thanks to user spielerinchen who pointed out that the last puzzle of Act 1 had bugs that could make it impossible to complete! It's fixed now.
To make a long story short, the security update to the underlying engine I recently did also changed the way the engine interacted with a third party tool I use for making the characters slide in to place. Some weird and mysterious interaction between two code bases that are not my own. (Lots of bugs are my fault, but this one didn't seem to be directly my doing). Anyways, it's 4 am, I've been working on this for 5 hours and I'm pretty sure I've totally fixed it and there will never ever be any more bugs. But you know where to find me if there are. Cheers!