It can be some technical detail.

For example: imagine you have 2 windows, the lower right corner of one window almost touching the upper right corner of the other, so that the bounding rectangles overlap but the graphics don't.

With the inaccurate "false square" corners, you just had to check the bounding rectangles, to know which window to resize, now you have to check the actual graphics (or more likely, a mask).

I am not saying it is the problem, but that's the kind of thing that can happen. Or it may be a simple bug, like a crash, memory corruption, an unhandled exception, the usual stuff, but they couldn't fix it in time and it is better to revert instead of leaving the buggy code or pushing an untested fix.

Most likely (and natural): they tested it publically and the response wasn't positive, so they held it back until they could do it better.

macOS does have weirdness with windows that span multiple screens. I bet some of that kicked in to an unacceptable level. It can create incoherent moving/snapping, for example. Has been kind of crazy-making for a while, for my set-up where screens are not joined but adjacent in a triangular configuration.

I think it shows how difficult it is to ship a seemingly easy thing inside the Apple machine.

I'm more interested in how or why this bug was approved up be worked on so quickly after it was surfaced, rather than other longstanding and arguably more impactful bugs.

Maybe it was just an oversight in the merge process? e.g. the diff was applied only to the RC and not to the release branch? idk

The AI reverted the change and no one does proper code reviews anymore so it went into prod.