The observatory is a seperate biome. By entering it, you make the creatures normally present in your biome E.g. ghost levithans in the void, think that you've left.

Try going out of your base long enough for him to respawn and deconstructing your observatory

Thanks @Pokemon was better ! Will try that.

You mean the observatory as the globe thing deconstruct?

If you are in a base in a biome, it counts as you being in the biome but the observatory is a seperate biome and it's probably better to just get rid of it to avoid accidentally walking into it and despawning your reaper buddy.

And yes, the observatory is the globe thing

He didn’t respawn yet.. I saw a vid about a Reaper Leviathan Obeservatory/base for inspiration. He too got an observatory attached to his base and yet in his case it didn’t despawn or dissapear.

Tried to command /spawn Reaperleviathan, but the one spawned don’t even make the scary sounds. A real bummer as the thrill and jump scares I get from these creatures falls away without any of them nearby. Whelp, guess I’m just fucked

Maybe restart your device

His video may have been on a subnautica game before the observatory was a seperate biome, or using cheats/mods that don't count it as a biome. Having an observatory isn't the problem, it's being inside it. The reason I told you to deconstruct it is to stop the reaper despawning again if you ever fix it.

I agree with Noah, it might be smart to restart your device

Will try that. Have deconstructed the observatory since yesterday and haven’t build it back ever since. Maybe a restart will fix the problem indeed. Anyway, thanks for the advice!