Maybe Chris already knows what you mean, but I didn't see anything near the top of the mantis list that looked like an obvious match. What new mouse behavior?
Look at the 'Wonky Mouse Controls' thread in the alpha forum, or on mantis.
Basically, the mech used to be fixed on-screen, causing the mouse to remain stable relative to the direction of the shot (same as in Nuclear Throne). Now, the mech moves all over the screen, and the mouse is fixed, meaning that your shot angle changes as you move. It makes it much harder to play with KB/M (IMO).
Mantis thread:
https://arcengames.com/mantisbt/view.php?id=18375
