While walking on soul sand that is next to lava you will be lit on fire regardless of whether or not you are touching the lava
Comments 3
I don't know if I should say that this is "fixed" (due to this actually being a feature request), but it no longer occurs in 1.16.0.59. See my comment on MCPE-45633.
This will likely be resolved as Cannot Reproduce.
This is due to Lava's hitbox being a full block regardless of the model currently being rendered. Soul sand is shorter than one block, so your feet are actually within the lava's hitbox.