That's expected, because once you mark a fuel-up as partial we need a full tank again to be able to calculate an MPG for an individual fuel-up. Fuelly works by calculating the difference from full, so without that full tank there's no way to get an MPG for an individual fuel-up. There's more info here and here. So the rule to keep in mind is that if you mark a tank a partial, you won't get an individual MPG rating for that tank or the next tank even if that next tank is full.
Keep in mind that all fuel-ups still contribute to your overall average MPG even if there is no individual MPG rating for a particular fuel-up.
I see, the fact was that i actually just topped up and brimmed it, so the last tank was actually measured from full in the normal way anyway. If i edit both fuel ups so they are both technically non-partial fuel ups, will they both show up then?
Yeah, Fuelly will do what you tell it to do. If you don't fuel all the way up but don't call it a partial you'll get an MPG rating for the tank. It won't be a realistic number, but you'll get a number. Because to Fuelly it will look like you went x miles and then it only took y gallons to fill it up again.