All Trial Spawners are empty in an old world

When I visit most trial chambers that are closer to spawn in my main (and quite old) survival world all of the trial spawners are always empty.

How to reproduce:

1. Locate a Trial chamber in the world
2. Search around it for the spawners

Expected result:
The spawners would have a mob floating inside and start spawning when you approach

Observed result:
The spawners are empty and do nothing in most trial chambers