Oh wow that’s odd. I’ve been trying to see if I can replicate the issue to no avail. I’ve been seeing a couple of threads where people have had the same issue and I’m thinking it’s possibly a bug. I’ll work up a proper submission and might need to get in contact with you again for a bit more specific info. In the meantime, I’ll link some of the threads with some of the workarounds the community had come up with.
Such as deleting all of the sub landscapes in the landscape tool first then going back to the outliner and deleting it there: Remove Landscape - #7 by The_Hivelord1
One member made an6 editor ultility blueprints to force delete items: UE5 World Partition can not delete landscape - #6 by Visnarel
Some suggest it’s the temporary level streaming assets causing the issue, but I can’t replicate that, but it involves saving the project, relaunching and looking for the streaming assets in the outliner and delete them first. UE5 World Partition can not delete landscape - #15 by PolyMad
Let me know if any of those help you out, and I’ll be doing more research on this side!