Make lots of frames until it reaches the right (aka when you have to scroll to the right to see the next frames)
make a tweening on any of the frames while the scroll is entirely to the left
then just create frames from the tweening and you'll see you can't scroll to the right until you do an action that updates the scroll length (like duplicating a frame)
Note: this also happens when you Ctrl + Z with all the scroll to the right (after reproducing this bug, just do an action that updates the scroll, then go all the way to the right, and press Ctrl + Z)
Steps to reproduce the bug: