Although the fuel system is "sealed" it must have path for vapour to be vented, and one for air to enter. In addition, there will be a safety pressure release valve. I'll ignore the filler for now, as it is normally sealed, except during filling.
The vapour vent path is via the canister and the air entry (I think) is via a non return valve, that only opens when the tank is at a negative pressure (vacuum).
The vapour vent is controlled with a solenoid valve, which, when opened by the ECU, purges the vapour (stored in the canister) back into the inlet path, utilising the vacuum in the inlet manifold. Since the volume of vapour that requires venting will be small (I think), the solenoid valve (N80) will not open very often, or for long. When it is open, and the purge process is active, a volume of fresh air, equivalent to the volume of vapour sucked into the inlet, will be drawn into the tank, via the non return valve. A vacuum will remain in the tank throughout, this is, (I think), important, because:
If the tank was vented to atmosphere (so not at a vacuum), the inlet would, effectively, have an air leak, (via the vent line/canister/N80
If there was a small leak to atmosphere via a worn filler cap, this would be most significant at low revs, where the volume of air via the leak would be high, compared with the volume of air flowing through the inlet, but not so at higher revs and higher inlet air flow.
This goes someway to an explanation of
@terrywindy1 observations, but not all the way. So far, the effect of the fuel cap leak would only be felt when the N80 valve was open. So, (I think) the N80 valve is not closing completely.. It's not unusual for a solenoid valve not to seal when closed, after many years of operation, the seat will wear and deform.
My guess is that the hesitation at low revs, on an FSI could be caused by the combination of a leaky fuel cap (or, indeed anywhere is the vent line), and a leaky N80 valve, resulting in a small air leak into the inlet.
Replacing the cap solves the problem, because with the system now leak free, a vacuum is maintained in the tank, so no air leak.
Consider replacing the N80, your FSI is worth it.
PS: I've found similar problems symptoms on the Mk1 TT Forum, and replacing the fuel cap is a fix. As all modern cars have these vapour canister/purge functions, it's probably quite common, but not much recognised.
Mac.