I consider this a major bug. It ruins any PvP server.
I am SURE this has been reported before, even by myself, but whatever the ending of previous tickets, this issue is STILL present and I find it may not be getting the attention it deserves.

The issue: Players sometimes get their base stats (NBT data) stuck on a specific value.

Example: say during a normal PvP, you are wearing an armor with a +10 health points attribute. Or a sword with 9 damage attribute.

There's somehow a slight (small) possibility that these attributes get stuck to your player's base stats (permanently modifying the NBT tags related to these attributes), and therefore, without wearing such mentioned attributed armors or weapons, your player inflicts 9 damage by punching, or has +10 health even without wearing an armor with bonus health.

This issue occurs seemingly randomly (for what I know), but there has been some reports on how to produce it. It cannot be undone by re-entering the server.

A possible temporal solution: If this is so hard to fix, (because i've seen this happen for more than a year now), please add an option for resetting player's NBT tags to default when they log out and back in, on the Spigot configuration file.