Scoreboard teams gets cleared when a player moves

Players are removed from their scoreboard team on my Spigot server as soon as they move (this includes changing the direction in which they look).

I run this command:

/scoreboard teams join TeamName @p

and then, without moving, this command:

/scoreboard teams list TeamName

I get listed as though I am in the team properly:

Showing 1 player(s) in team TeamName:
<my username>

Then, I'll move my player, and run the same command again:

/scoreboard teams list TeamName

I get a message saying the following:

Team TeamName has no players

The above^ also happens if I add myself to the team, move, and then list the players in the team. It only lists me until I move. It happens to all teams.

I should also note that one of the plugins appears to be creating a team named collision, which automatically adds players to it. It re-appears even after I remove it. I have no clue as to which plugin is creating this team, and have no way to tell without deleting each plugin individually.

Server Plugins (1.12, run on Aternos):

• OldPvP (maybe this one?)
• BloodEffects (maybe this one?)
• Citizens
• InstantRespawn
• Multiverse-Core
• NickNames (maybe this one?)
• WorldEdit
• WorldGuard

Solution 1:

I've figured it out. The OldPvP plugin was overwriting the scoreboard teams, causing the issue. I've deleted the plugin and installed OldCombatMechanics instead; works like a charm.