While I have a fuel filter on my '38 it's not a see through filter BUT this reminds me of my old '70 Harley that I put a see through filter on. It was never full of fuel but the bike ran normally. I puzzled over it for a while but since it ran OK I never pursued the "why". It wasn't a problem so there really wasn't anything to "fix".

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.