A rewrite is Google deciding your title tag is not the best description of the page and swapping in text it judges fits better. It is a relevance and clarity call, not a bug and not a penalty. Google has confirmed titles still count toward ranking, and pages with rewritten titles do not lose rankings because of the rewrite. So the swap is not punishment; it is Google’s read that your title and the page were not quite saying the same thing.
In its own announcement of the change, Google said it now leans on the visible text of the page to build titles, especially the main heading, the H1 and other prominent headings, and sometimes the anchor text of links pointing to the page. It reaches for those mainly when the title tag is too long, stuffed with keywords, generic boilerplate like “Home,” or simply mismatched with what the page is actually about. The HTML title is still used the large majority of the time; the rewrite kicks in when something about it does not fit. The most common substitution is pulling in the H1, because the H1 usually reflects the topic a visitor actually sees.
That makes a rewrite a useful signal rather than a mystery. It is telling you that your title did not match the page, or did not match the query the page ranks for, as well as the text Google chose instead. If Google keeps swapping in your H1, your title was probably promising something the page does not lead with.
So when you see a rewritten title, do not fight to force the original back. Check whether your title honestly describes the page and stays close in meaning to the H1 and the query it ranks for. Tighten the alignment between the title, the heading, and the content, and Google has far less reason to override you.