I have noticed that some instances of the EntityPotionEffectEvent are missing some causes:

BeeSting - When a player is stung by a bee they get the poison effect, the cause returns as "unknown"

Patrol Captain - killing a patrol captain gives the player bad omen and returns "unknown"

Wither rose - when an entity steps into a wither rose, they get the wither effect, returns "unknown"

Added my tester plugin, to replicate:

1) Get stung by a bee -> message will send "UNKNOWN"

2) Kill a patrol captain -> message will send "UNKNOWN"

3) Walk thru a with rose -> message will send "UNKNOWN"

PRs to fix this issue:

Bukkit -> https://hub.spigotmc.org/stash/projects/SPIGOT/repos/bukkit/pull-requests/470/overview

CB -> https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/pull-requests/620/overview