OK I've been able to reproduce it!
In Unity 6, I've got a canvas with two children in descending order: HexContainer, QuadText.
HexContainer contains 1000 gameobjects which have a rectTransform and Image component each.
QuadText has a SuperTextMesh object with some text and the default clap quad on it.
The moment HexContainer has 1024 children, the quad vanishes from STM. I need just a bit more time to think about this, but I have a repro scene now so this is great.
Like it's pretty strange... it's *not* a material count issue like I suspected, the moment this condition is met, the actual geometry of the mesh changes! Trying to figure out if this is linked to submeshes... I wonder if this could *somehow* be related to a bug Unity 2019 had I was never able to solve where quads just wouldn't render on canvases in some versions? The end result is similar... Submesh Materials are still being generated, but not rendering at all so it might be something with submeshes...
Still thinking on this, just wanted to post to give an update!!