I have been having an issue with blank surface guis on mobile recently. It does not seem to apply to all surface guis. From my testing it only happens to surface guis that are cloned and parented into workspace, not ones that existed when the player joined the game.
Below is an example of a surface guis with text on it that is blank. Oddly enough it appears as though you can see the text stroke of the text that was on the surface gui.
