It is from calling SetImage, I would recommend working around this bug for now and explicitly not using SetImage anywhere. I find SetImage causes this white block issue to happen inconsistently, Creating → Drawing → Destroying → Creating → Drawing → Destroying → Repeat causes the material_block to show correctly randomly or incorrectly randomly.
I know sometimes you want to change the underlying material as this was my first approach at using material_block, But currently I save the material and set the parameters for example if I need to change the texture I parameterize the texture in the material graph and just call set SomeMat.SomeParamName = SomeAsset
This does make your materials more complicated then they need to be of course, But seems to at least be consistent.
There are a few bugs I noticed, Not just with material_block but with material assets as well for example: Calling Print(“Some Text”) anytime from anywhere in any class before changing a parameterized material parameter with set causes roll back and you can see it in the log. I didnt notice this pre-37.00 so if you get roll back that could potentially be why.
Edit: Also I found that drawing a material block directly in a canvas slot or in a parent container with no other children elements causes the block to turn white randomly, Even with the work around above. There are a few spots where I actually just use a hidden element in a stack_box with the material_block and it behaves.