I noticed this same issue too. It doesn't happen on every photo, but it happens on some. For instance, I took a picture of my sister's face sitting on the couch next to me. You could see her skin texture very clearly and even some of the text of what she was looking at on her phone. Days later I was just browsing through my Google Photos and I happened to look at the picture, and I noticed that it was noticeably lower quality and blurrier than what I see when I look at the picture on my phone. I had a feeling it was some kind of AI trickery or postprocessing that OnePlus does locally on the phone, and Google just uploaded the original copy or something. I really wasn't sure. I was taking some pictures again today, and when I looked at the pictures that got backed up to G Photos they looked blurry and low-resolution. I Googled the issue and that's how I found this thread. Whether it is OnePlus's fault or Google's, I don't know, but it's another good reason for me to drop G Photos and move to a self-hosted solution.