Google's John Mueller said on Twitter that Google rewriting your page titles is not a sign of a quality issue with your page or site. He said, some pages have good titles, some have bad - that is just the web. I am sure my title here could be better, so Google have at it.
Lily Ray asked the question, asking "if Google is overwriting your article titles at scale (say, 60% of your articles) - could this be perceived as a potential sitewide quality issue?" It is a good question that I never thought of before.
But John Mueller of Google said nope, he said "I wouldn't consider most of those to be quality signals. Good sites have bad title elements, bad sites have good title elements."
Here are those tweets in context:
Looking at https://t.co/H3qKn8c4uK while some of the items seem kinda quality-related (keyword stuffing), independently of Google, I wouldn't consider most of those to be quality signals. Good sites have bad title elements, bad sites have good title elements.
— johnmu 🐀 (@JohnMu) November 2, 2022
So don't panic if Google is rewriting your titles, at least, not from a quality issue or ranking issue. It might impact your CTR or your messaging, and then you can panic. But in terms of Google not trusting your site, or thinking less of your site, it does not seem you need to worry about that.
Forum discussion at Twitter.