are you using the same liquid in your other Heron? I need slightly more fluff with my 50/50 liquids than I do with my 100Vg Ones.
You can have too much fluff but still could still be over filling it. I had that when first using my Spheroid. I could only get a little over 1mL of liquid but when turning the tank over (without the base) eliquid would drip out to the point where it would drip down the center tube (even after a bit of
vaping).
If you can't get at least 3mL in an empty Heron tank, or 2mL in a Spheroid, you have too much fluff.
If you have liquid dripping out of the tank when inverted, you have over filled - no matter how little, or a lot, of fluff you have.
The Herons and new Spheroid are a bit more flood resistant since the raised center holes give it a bit of a drip well...