I am consistently shocked at how bad Apple Notes is. I like the features it has and I like the iCloud sync between devices... I use it daily. But it's so damn buggy. It's a notes app! Why do new lines on a moderately-long note cause half the text to disappear until I hit the carriage return a few times? I also wish it simply converted copied text to plaintext by default rather than trying to preserve formatting.
I have three notes that exhibit the bug you mention though: the three notes I keep for each of my children's artwork. I scan the artwork using the document scanning tool in Notes, and it gets embedded as a multi-page PDF (if the artwork itself has multiple parts) or a single PDF. After many years of adding high-res scans, when I scroll to the bottom of these files it takes some time for the note to render. I think I picked the wrong tool for the job here, more than anything!