> a long-standing bug was accidentally fixed
In some cases (e.g. in our case) long standing bugs become part of the API that customers rely on.
In some cases (e.g. in our case) long standing bugs become part of the API that customers rely on.
Obligatory: https://xkcd.com/1172/
It isn't actually possible for fixing a bug to break a workaround. The point of a workaround is that you're not doing the thing that's broken; when that thing is fixed, your flow won't be affected, because you weren't doing it anyway.
That's not true. For instance, if there's a bug in formatting, that might be worked around by handing the unintended formatting. But now you're (maybe) not handling the intended formatting, and a fix would break you.
Maybe it's a bad workaround, but your users will almost certainly implement bad workarounds.