This issue has changed shape in the last few months and is now more constant.
The realm is set to add new players as Visitor. They all join using the supplied code. All new members are actually joined as members and have member access to the realm. At some unknown point, often the next day, the players are switched to Visitor. I know this as I have had many new players recently and they all come to me asking why they have been switched to visitor mode. I explain that it is a bug. This happens to every new member so I can reproduce this.
@Tman Bacon. it is not just in the fortress. It just so happens that I made tot a fortress.. which took a long time. I'll do another recording outside the nether portal. It's just not right.
You can skip the first minute of magma cubes: https://youtu.be/lMGuyJaXxnU
Tman Bacon, I've added a 5 minute recording (link) so people can see how fast the spawns are. It's not the best as I have protected the area a bit, but it should demo how fast things come back. I have not changed any settings from the default.
Edit to add an a link to a recording.
version 1.19.81 on Realms
Normal difficulty
Spawning in the Nether is a mess and makes the nether almost unexplorable. All types of mobs can spawn immediately after previous ones have been killed. For example, clearing mobs in an open area, turning away for a few second and turning back will often result in the mobs having respawned. Ghasts, blazes, Wither skeletons, all types. The state it is currently in makes any mob spawners irrelevant.
The expected behaviour is that when mobs are cleared, there will be a gap between the next mobs spawning.
The setting is controlled by a gamerule. Game rules reset after 20 minutes of there being no one on the Realm. If you have cheats enabled then you can set this with command blocks. I have a series of command blocks (in a ticking area) setting this and more every 20 minutes (using a tick delay). Good luck.