As of about December 2018, I found that SurfaceGuis are not rendered at all in the Roblox mobile app on my device. My phone model is the Samsung J7 Prime. This is on the latest version of the Roblox App at the time of writing (Feb 8).
The comparison is shown as below. Above is what it is meant to look like, below is what it looks like on the device mentioned above.
(A perhaps similar problem can be found in another post from Sep 2017: https://devforum.roblox.com/t/blank-surface-guis-on-mobile/52789)