That's sometimes unavoidable. If there's a clear bug somewhere, fixing it will sometimes also affect queries/plans that weren't visibly affected by the bug. That's especially the case around costing issues - some queries might have regressed noticeably due to a bug since the last major version, but fixing it might affect other queries negatively (in a minor version). Postgres/we try to avoid that, but sometimes that's the most sensible way forward (and I assume the same is true for mysql etc).
