The FlipBookUV node is broken for non-square layouts since internally it uses the wrong calculation for the rows and divides by Y instead of X.
So for symmetrical layouts where X = Y it works, but as soon as you have more rows than columns, the bug becomes obvious.
This is how it’s currently, dividing through the number of rows instead of the number of columns:
This is how it would need to be to work correctly in all aspect ratios, dividing by the number of columns instead:
See this (epic) thread for example scenes and background and other issues:
