I would like to say that this bug isn't totally related to the GUI interface being hidden. Because I'm 100% sure that I didn't have the Hide GUI option on when this happened. See details below.

I believe this might have to do with the game being too laggy on a certain chunk/ having too many blocks of a certain kind. I have yet to prove the latter.

What happened to me was, I was placing down jack o' lanterns while my mcpe app has been on for about 30+ mins. After placing a jack o' lantern down, suddenly the game was glitchy and then the GUI disappeared as well as the screen was flashing in colors of blue, purple and white like as if the ram of my device was so used up it couldn't load any more blocks. It was like having a broken/low end video card while playing a high end graphic game. I even restarted my phone and the bug was still there.

Even while the GUI is hidden, I could still walk around and while walking there were certain instances where the blocks would just seem to disappear like I was in xray mode but they were still functional.
I thought at first it just had something to do with the ram on my phone for playing for a while. When I had cleaned up the apps using both the standard app cleaner my phone has and an app killer program it would get fixed. As soon as I logged back in the GUI was still missing. I also found out that it took down/removed the texture pack I was using and had to add it again from the settings menu on the game home screen. (I was using the natural texture pack)

I could confirm this has to do with lag/block at a certain chunk because when I had walked out of the zone where the jack o' lantern was placed, approximately 4-5 chunks, the GUI appeared after restarting the app.
It also has something to do with refreshing the app as when I open it initially, there was no GUI. But when I go to the home screen and then log back in while the game was running in the background, It appeared for a few seconds and then was gone a few seconds after.

What I did to fix this bug was, since I recall the location of the Jack o' lantern I placed that got me bugged, I took it down and then reloaded the game. Fortunately for me the GUI came back. The missing GUI also applies to the menu screen and anything else that has to do with any interfaces. I couldn't even see the quit game, settings and other tabs window when this happened. Literally the whole GUI was missing.

I've found a few bugs posted here that are related.

Here's the ticket numbers for all three related bug reports:

MCPE-17921 - Gui Vanishes
MCPE-16943 - GUI broke/glitch after place more than 144 couldron in player render range.
MCPE-18024 - Minecraft Pocket Edition 0.16.1 no hud

My device is a lenovo A536.

I had to repost this because the other 3 bugs were deemed unconfirmed bugs without any resolution and reply from the users too.