Mobs like skeletons don't burn while on soul sand. Mostly likely a result of MCPE-12719.
To reproduce spawn and skeleton on soul sand during the day. It starts to burn as it is spawned just above the soul sand, once the fire goes out it will not restart even though it is still in the sun.
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Welp they were burning then they stopped.

This seems to be fixed in 0.15.90.2
Cannot reproduce - I'm assuming this has been fixed.