This is difficult to explain, but when a banner exists that is not rendering, any block update of placing will change the unrendered banners to look like the block placed.
See video for visual explanation
https://www.youtube.com/watch?v=PzV0WHb-P8c
All these issues were in the same chunk. Unsure if reproducible without excessive banners.
I changed the world layout and can no longer test this bug.
As I may be one of the only ones who have experience this, I'd say you can close this as "Cannot Reproduce".
I assume MC-87802 fixed this. I will request a reopen if I encounter it again.