This glitch is hard to explain, first it only work if it's lava that create the fire.
Setup
First place the block that you want (liquid and air INCLUDED) then place unflammable block around and lava, place flammable block over the block that you chose (but not over the bock directly) then wait. If the fire has spread on the flammable block over the block you have chose, you will see the fire like if it was on a normal block
The images show the setup and 8 strange fire spreading
Edit: In the new version (20w09a) it doesn't work everytime but SOMETIME it does the glitch
Edit 2: Bug is accidentally fixed in 20w10a (bug is NOT necessarily fixed, but the old setup doesn't work anymore)
I see that it's resolve because of Cannot Reproduced but, it's normal, IT DOESN'T WORK on 20w10a and later so it's normal that you can't reproduced in those version