Background: 4.0.3. A 3D "fps" world, with the usual capsule shape suspect protagonist. I'm adding a SubViewport, and a Camera to render onto a Decal with albedo set to a ViewportTexture from the SubViewport.
I can (sort of, unreliably) make it work in the editor, but never in the game(!) With transparent bg, it's just invisible, without, it's black, and if I pick the icon.svg instead, that works. The (inner, well, both actually) camera is active, and I've fiddled with Clear Mode and Update Mode on the SubViewport. (and every other setting I can think of)
In the editor, rendering both camera view and a label on top works, both in the SubViewport texture view, and on the Decal in the 3D view(!)
I... discovered that it works if I use a plane mesh and just position it manually. The conclusion then has to be that there is something screwy with how Decals get their textures. Maybe there's even a bug involved? Should I try to make a minimal repro?