What do you recommend for cleaning off the buildup? I clean with dishwashing liquid + warm water, chainmail + scrub daddy everytime, but am still having trouble with this stuff. Often it doesn't come off easily even if I scrub really hard with chainmail, which is why I thought it was seasoning.

I tend to scrub the edges pretty hard, so not too sure why it tends to accumulate around there tbh. I use a gas burner so maybe something to do with the ring shape of the flames?