Applies to Warped Stems, Crimson Stems, Crimson Roots, Stripped Crimson Stems, and Stripped Warped Stems.
This does not apply to Warped Roots, Nether Sprouts, Weeping Vines, or Warped/Crimson Planks, Stairs, Slabs, Fence Gates, Fences, Doors, Trapdoors, Buttons, Pressure Plates or Signs.

When one of the above mentioned blocks is near lava, it will cause fire to spread beside or ontop of the block as if it were a flammable block, despite all five blocks being non-flammable.

To test this, plant a crimson root, or place a stem within fire-spreading range of lava. (For details on this range, see https://minecraft.gamepedia.com/Lava#Fire_spread) with blocks beside it to spread onto. Set the randomTickSpeed gamerule to a high value (I used 3000).