Same issues here. Had a 12 Pro, updated to iOS 18, immediately noticed that changing text color in markup wouldn’t work, almost as if the color screen was force closing. It fixed itself for a couple days, then the same issue came back. My husband has a 14 Pro, my son has a 14, and their markup works just fine. I just upgraded to a 16 Pro since I was eligible for an upgrade and my 12 Pro became so laggy. I have the same issues with markup with my 16 Pro as I did with my 12 Pro, so it’s not a phone issue. The issue seems very user specific, but I do think it’s an issue with iCloud. I can save a photo to my files, open up the photo within the files app, and markup works just fine. It does not work through the photo app.