A defect that was fixed months ago reappearing is a common and frustrating pattern. It usually indicates that the original fix addressed the symptom without changing what allowed it.
A fix without a test is temporary
Code changes constantly, and any behaviour not verified automatically depends on nobody accidentally undoing it. Over enough changes, someone will.
A test written alongside the fix converts the fix into a constraint the codebase must keep satisfying. Without it, the correction is just one more line that can be refactored away.
This is why mature projects treat a regression test as part of the fix rather than as follow-up work, since the fix alone has a short expected life.
Merging can silently revert work
When several branches are developed in parallel, a fix on one branch can be lost if another branch based on older code is merged afterwards.
Version control resolves textual conflicts, not semantic ones. If the two branches touched different lines, the merge succeeds and the behaviour is quietly restored to the broken version.
Long-lived branches make this far more likely, which is one practical reason teams favour frequent integration of small changes.
The same mistake gets made again
Many defects arise from a pattern that is easy to get wrong, such as an ordering assumption, an unhandled edge case or an interface that invites misuse.
Fixing one instance leaves the pattern intact, so the same class of bug appears elsewhere and looks like a recurrence. It is a repetition rather than a return.
Durable fixes therefore change the shape of the code so the mistake becomes difficult, rather than correcting the single place it appeared.
Intermittent faults are never confirmed fixed
Defects depending on timing, ordering or load may not reproduce reliably, so a change that appears to resolve one may only have altered the probability.
Because the failure was already rare, its absence proves little. The bug is closed on the basis of not having been seen recently.
Environment changes reopen old ground
A fix may depend on behaviour of a library, operating system or compiler that later changes. The code is untouched and the defect returns anyway.
This is common with workarounds written for a specific version, since a workaround assumes a condition that will eventually stop holding.
Recording why a change was made, and against which version, is what allows the next engineer to recognise a returning defect instead of investigating it from the start again.