We need more details to help: which version do you fish on? Does it also happen on singleplayer? Are you using any mods?
If do you mean a Java bug: Report it here: https://reports.cubecraft.net/bug_report If do you mean a bedrock bug: Report it here: Bedrock Player Reports Please report players on our Bedrock network here. www.cubecraft.net
I play on switch no mods or packs etc when I fish the bobber goes up and down VERY fast I've rested my switch and everything
Ive got that bug a few times its just a visual bug it doesnt affect ur fishing just make sure the water is all source blocks