No. You can't polish out stain. It's absorbed inside the paint. DO NOT polish matte paint unless you want it to look glossy.
However, what I have found is sometimes bug juice that can get absorbed in the paint and a good way to get it out is to let the sun bake the bike in the heat should be able to get the stain out or you can try a hair dryer.

 

@Paddlefoot64 Poss to post a pic?

 

I've had the same stain on my white SSS, and used Dr Beasley matte paint cleanser, letting it soak a few mins, then it wiped away. The stain came from not fully clicking the filler cap down after filling up.

 

As others mentioned, can't use any kind of compound or polish. I think Boe has an answer. Alternatively, my painter suggests mineral spirits for paint stains. I'd try a hidden sample area before using it in a visible area. FWIW, I also use Chemical Guys Matte cleaner spray and their sealer, never had a problem. Sounds like gas stain.

 

I will try to get a photo. Thankfully, the stain is subtle. I’ve used mineral spirits to clean painted wheels before. I need to buy some to try on matte paint.

 

Does it look like this?

 

Above does look like a drip stain, to me. Mine is very subtle, but in similar location.

 

Seems like the cause is the same... not fully closing the cap until it clicks. And if I could clean mine off with the Dr Beasley paint cleanser you must be able to find something that works. No luck yet?

Also, I pulled the filler cap apart to check the seal and cleaned it all up inside the lip with a rag and the same cleanser. No drips since.

 

I heard that some SS have the problem of loosing gas through the lid when riding downhill and braking.

 

Never had an issue, I’ve owned since early 2018.

Make sure you hear the cap click when you close it. If it gets sticky a spray of WD40 does wonders.

 

Cap won’t close until locked with key, on mine, so not issue of riding while not shut