I have this situation with the Opportunities module. We have a 2 subpanel custom on the create view of opportunity. This works fine on the 11.3.0 version.
We have a clone instance with version 12.0.0 where these subpanels are no longer visible in the creation view. They are only visible in the Edit view of the opportunity.
Has anyone gone through something similar? Any idea about what may be conditioning the custom?