Came across several random invisible blocks. When I was placing grass blocks, they kept going above where I was tapping, just random places in the air. When I held down on the spots where it wouldn't let me place anything, it reacted as though I was cutting away a stone block of some sort (sounds like stone blocks being broken, animation of breaking a block). After I do that, I'm able to place blocks in those places, like they were just invisible stone type blocks. This is in an area with only grass blocks placed manually though, so couldn't possibly be stone just not showing up.

Screenshots included to show where blocks were being placed, even though I was only tapping on one specific block on the ground (2 examples included).

HTC U11 being used.