Hi and welcome to the forum
Yes you are still entitled to a repair under warranty unless you have voided it somehow, for example getting it wet.
2.1 The Fairphone smartphone device, the separately bought Fairphone accessories and replaceable smartphone modules (“Modules”) are covered by this Warranty for 24 months from the date of delivery to the original purchaser.
Fairphone 3 Warranty - What is a warranty and how does it work?
For current Fairphone owners: Use these parts to repair your phone only if you’re sure that the damage is not due to a manufacturing defect. In other words, be certain that your phone is out of warranty before you attempt any repairs by yourself. If you’re in doubt, contact us before you order
Spare Parts | Fairphone
Did you notice any discolouration of the dampness indicators, the red square in the image below is an example of getting wet, even a pink colour would void the warranty.
Well Android 11 is now the default for FP3 and FP3+ not that A10 indicates an issue, but support will no doubt ask you to update as one of their steps in ensuring your are on the same page as them.
Also try to charge without a SIM and SD card.
It could well be the USB port which isn’t very easy to clean
See this topic and note the last post