If theres fire at the side of a block, and you place soul soil beneath it, the flame will turn blue and be fueled by the soulsoil. once you remove the soul soil the flame is gone.

This behaviour only happens with blue fire. if you put a normal burning block under a flame attached of the side of the block, the flame will just jump back to the side once breaking the block beneath

As seen in this video https://streamable.com/caunq