If you dig 2 blocks down into a flat area in a body of water, place a magma block, it will not create bubbles unless you place a water source ontop with a water bucket. If a piston (or player) were to then place a block on top of this magma block, it will delete the source and it will not create bubbles. This does not happen on Bedrock edition, so i think it is a bug. This also happens with soul sand blocks.