When playing in a world that was created in a Java Realm with the Update 1.21 experiment applied, before version 23w45a, naturally generated trial spawners in trial chambers are deactivated and do not spawn any mobs. Image provided.
Trial spawners also do not spawn any mobs even when using a mob spawn egg on them, both in Survival and Creative modes.
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This issue can be closed. I have found it was the doMobSpawning gamerule being set to false causing the trial spawners to not spawn mobs.
There is a Java client bug report though for the doMobSpawning gamerule thing: MC-266487
Upon further testing, this issue only affects worlds created in Realms before version 23w45a. When creating a Realms world in version 23w45a, the trial spawner works.