It is not possible really to have a heavily modded snapshot server. I've not tested it on a new world. The server is running on vanilla jars. This has never happened before when updating through several versions of 1.16 snapshots.
This is an an existing server running over several versions of 1.16 snapshot.
It's running on a Ubuntu server. Fabric has been used previously but this issue appeared on vanilla jars.
Server was updated from previous snapshot where the dimension IDs were as expected. Now dimension IDs are returned as described above.
I'm not able to do that. Hopefully if anyone else has a similar issue they will find this and can report.