Not sure if this is a common problem, but the first time I've had it happen.
Last couple of fillings I've noticed that the amount released after decoupling the filler gun seemed a lot more than usual. Had a look and was going to change the filler point (but then discovered the only ones I had were for 8mm hose not JIC) but then noticed it looked like something was in there that shouldn't have been!
We have now learnt that you can undo the centre bit of the filler (first time I've had one apart) by unscrewing it. I'd guess a proper tool exists, but we sacraficed an odd 13mm socket to to the angle grinder to sort it.
Can only imagine some helpful soul had stuffed it into the end of the filler gun, it looks like the end of the packet off a straw to me.
Hopefully, this might also cure the slow filling I'd been having. If it doesn't then the next thing might be to undo the rest of the hose and check there isn't any more of it hidden in there!