Textures of most blocks turns weird after you close minecraft (with the middle button), and then open it up again by the "running in background" tabs menu.
Doesn't happen everytime, but has happened a lot of times.
Tools and mobs don't seem to be affected by this bug.
Thank you for your report!
We're tracking this issue in MCPE-159115, so this ticket is being resolved and linked as a duplicate.
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