Furnaces Facing North and East have only "Flame Particles" in the front side of the furnace , while South and West have their "Flame Particles" in their Back Side of the Furnace

While am on my survival world , I thought flame particles didn't been added yet , because I placed it facing whether south or west ,which have their flame particles in the back side of the furnace.

then I noticed that it was been already added when I put the furnace facing whether North and East, which is has its flame particles in the front side of the furnace