Topic category: Bugs and solutions
There's a bug: adding a custom overlay causes many transparent overlays lose its transparency, for example the hotbar or the purple nether portal overlay. It is an annoying issue, because when you walk into the portal you stop seeing everything.
The solution is to change HELMET in
event.getType() == RenderGameOverlayEvent.ElementType.HELMET
in the code of this element to TEXT which will make the elements render properly.
I am on MCreator 2020.4 making a Forge mod for 1.15.2