The bug
I moved from a glass block to a soul sand block while crouching adjacent to lava. I knew I would catch on fire because of the 7/8 hotbox, but I also fell off, even though I was crouching.
How to reproduce
Place down any normal height block adjacent to lava, crouch on the edge of that block
While crouching, move to soul sand which is also adjacent to lava
I think that is because technically, you fall for that 1/8th of a block distance down... and while falling moving also slightly outwards (towards lava). Just enough to get a tiny bit over the edge and the sneaking protection no longer works for that soul sand block.