I might have the same or a similar problem with 26.50 on Minecraft PE (iPad). As soon as I tab “Play“, I can only see the background where you would normally see all of your worlds. When I tab on the screen, I can join worlds (so the buttons are there, but just invisible), however, in-game-buttons are also invisible (i. e. Jump button, walk directions, pause the game, …). See more in the screenshots
Sectimon
I might have the same or a similar problem with 26.50 on Minecraft PE (iPad). As soon as I tab “Play“, I can only see the background where you would normally see all of your worlds. When I tab on the screen, I can join worlds (so the buttons are there, but just invisible), however, in-game-buttons are also invisible (i. e. Jump button, walk directions, pause the game, …). See more in the screenshots