Hi!
In many cases, the coil’s cotton is insufficient; it simply lets even 80/20 liquid pass through. The Smok RPM3 coil suffers from a similar teething problem; I have a Smok Nord 5.
It often happened that it leaked after filling, but since then I have learned to disassemble the coil. It has 6 layers stacked on top of each other. I modified it by assembling it with 8 layers: there are 6 layers inside the mesh sheet, and 2 layers outside, which results in better liquid retention.