Hello .Driehuis,
After speaking with the developers the it appears that the report that was written up was actually working as intended. User Interface domain materials may only be used with slate and UMG. They are not intended to be used with Canvas. As for the issue that you are seeing. I was unable to reproduce this issue on our end with the information provided. I noticed that you said you are continuing to look into this issue for a reliable reproduction. When you have this worked out, could you reply back with a detailed list of steps to reproduce this issue on our end? Could you also include screen shots of any settings/blueprints that may be involved?