I am using a Subtank Mini with a 0.5ohm coil (tried 3 different ones so it isnt the coil) and at 18-20w, the warning sign (over the ohm reading) flashes when I try to vape above 3.2v.
3.2 x 3.2 = 10.24 / 0.5 = 20.48W, which exceeds the 20W limit. There's a little play in the math due to the 0.5 coil not being exactly 0.5 - could be a little higher or a little lower, and to make it more complicated, coil resistance changes based on temperature as well.
You're hitting the wattage limit.