I'm playing in snapshot 20w11a, and it's impossible to get projectile protection. Either by fishing or by enchanting by yourself. I play on an online server with 10 friends and no one has any form of projectile protection so far.
I don't know if this is fixed because we don't want to update the server because of the fishing update just yet
Can you reproduce the issue in the latest version (20w12a)? It may have already been patched out.