Just like the flail from before. I also think the musket could use more durability because 80 is quite low for it. Maybe the type of bayonet used can also affect the durability or there could be another way to upgrade the guns?

Probably hotfix-time once more since we already have 4 new bug reports :D

I investigated a bit further and found out that this issue is only half-correct :D
The musket is actually damaged - however, the damage value is separate from the "overall damage" value.
The bayonet damage is counted individually in a NBT tag. When the underlying knife breaks, the musket remains in your inventory - and vice versa.
However, I do agree that it would be a good idea to also somehow indicate the bayonet damage (in the tooltip, for example).