Soul fire can't be created when placed on soul sand, the fire becomes regular fire.
MCPE-66083
Placing fire on soul sand does not generate soul fire
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In the nether update, soil fire is only created by lighting soul soil on fire. lighting soul sand on fire will produce normal fire like before. Soul Soil is native to the Soul sand valley whilst Soul Sand is prevalent all over the nether. You probably mixed up the two, so I hope this helps you.