I have a Minecraft server that is about 5 years old and it seems Trial Chambers may be bugged in older worlds like this.

Trial Spawners have given no loot at all and I've gone through around 10 or so Trial Chambers. All the chests and pots in the trial chambers were correct, and the pots were the only ways to find Trial Keys if I was lucky, considering the spawners weren't giving me anything.

I even made a new singleplayer world and tried it and it works just fine in the new world. 

I even went out tens of thousands of blocks just to be sure it wasn't just a bug with already generated chunks.