The cameras on the Pro versions used to be so good, then starting around one-plus 9 they really went downhill. I have photos in my archive from my OP8 Pro that look much better than those taken with the OP13.

Coinciding with Samsung nerfing the Ultra (aside from the bloatware) - it's not looking like a great landscape for Android Phones.

Anecdotally, the OnePlus 7 Pro had decent but super fun cameras. Some color issues.

My OnePlus Pro 9 felt like a good step up in photo quality.

And my OnePlus 12 takes awesome photos I'm very happy with. I didn't experience the same "downhill" as you.