The Bug
The fishing line becomes increasingly thinner the closer to the rod it is, to the point where it is no longer visible at all.
I know that others do not have this bug, so I assume it's related to my GPU most likely.
I have put the most relevant parts of my PC's setup into the environment. There is also a manually triggered crash report in the attachments.
This was not an issue in 1.16.5, but is also not new in 16a, as I could reproduce it in 15a as well. I might try to pin down the exact version if neccessary.
Standing on the ground:
[media]
Flying in creative (line seems detached more, likely MC-6579, but shows it more clearly)
[media]
How it looks for me in 1.16.5 (expected behaviour):
[media]
can confirm