Not sure if this is a The Beneath or Advanced Rocketry problem. But the dim gets created as a moon, even if I change the dimension id to something different it gets recreated as a moon. Requires oxygen and doesnt create propper.

The Beneath-1.12.2-1.3.0
AdvancedRocketry-1.12.2-1.2.6-27

That's Advanced Rocketry. I do believe someone has already reported it to the devs, and there may already be a fix out for it, but that mod somehow overrides dimension IDs (or simply has a fixed range it assumes are planets). There was some mention of it over at AllTheMods/ATM-3#291, where by the looks of it the thing comes down to configuration (don't quote me on that, I only skimmed through the conversation).