I can’t speak to Terraria specifically, but just with most games in general?

An actual bugfix often means you changed the underlying guts of some system, some system that had appeared to work, but actually, had a fundamental flaw, even if minor.

With that kind of a fix… well, often mods essentially work by creating their own uses of, or work-arounds for the old buggy system.

So, when that system is reworked, to remove the bug… well now some of the guts of the mod may need to be reworked as well.

An extreme example of this would be the FO4 ‘update’.

Not really much in the way of new content… but they reworked a bunch of the guts. Which broke just most mods that did anything beyond something entirely simplistic and superficial.