On a multiplayer server, trial chambers are over spawning mobs beyond what is expected. On a server that I play on we had over 100 persistent skeletons in one trial chamber and 280 persistent skeletons in another. No server side resource packs or behavior packs are installed. I've seen at least 3 other instances of complaints via discord about trial chamber over spawns that suppress normal mob spawning when mob cap is exceeded.

It appears that once a trial is activated and not completed, then the trial spawner(s) continues to generate persistent mobs when new players load the chunks containing the trial spawners. New players are not entering the trial chamber to cause these mobs to spawn. New players are simply walking over the chunk containing the spawner. Over time with multiple players moving over the same chunk containing multiple spawner, excessive persistent mobs build up and exceed mob cap. Slimes are particularly problematic as large slimes break down from fall damage into a larger quantity of smaller persistent mobs.

Steps to reproduce

1 Create a world with seed -7592770070013269424, coordinates on, all other settings default, and creative mode for ease of travel.
2 /tp @s 179 6 114
3 Switch to survival mode, look at breeze spawner and stray spawner to activate trial. Note that 3 strays and 1 breeze spawns
4 Switch to creative mode
5 /tp @s 179 100 114
6 /execute as @e[type = breeze] run say hi
7 /execute as @e[type = stray] run say hi
8 Note that 3 stray and one breeze says hi
9 /tp @s 0 100 0
10 Have second user join game
11 /tp [2nd user] 179 100 114
12 /tp @s 179 100 114
13 /execute as @e[type = breeze] run say hi
14 /execute as @e[type = stray] run say hi
15 Note that now 1 breeze and 6 strays say hi

Expected behavior
Trial chamber spawners should time out and despawn mobs. Like a raid timer, if a user does not complete the trial before the timer runs out then the trial fails and does not generate loot/keys. Any mob within simulation distance of that spawner mob type will have their persistent flag set to no and then normally despawn. If a player loses items due to death and then a mob picks up that item, then the mob will be remain persistent until killed.