Trial spawners don't work for existing Snapshot Java Realms worlds
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.
Environment
Java Realms - 23w45a
Created Issue:
Trial spawners don't work in Snapshot Java Realms
When playing in a world that was created in a Java Realm, 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.
Environment
Java Realms - 23w45a
When playing in a world that was created in a Java Realm with the Update 1.21 experiment applied, 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.
Trial spawners don't work for existing Snapshot Java Realms worlds
When playing in a world that was created in a Java Realm with the Update 1.21 experiment applied, 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.
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.
relates to
duplicates

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.
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