I have an FP6 running under Android 15 (latest release). I have noticed that when charging only I have problem keying in stuff, e.g. password. Is this a normal state of affairs? Or a bug? A usual behaviour is that one press on the screen produced several numbers (repeat) which messes up the password entry.
(With other previous phones, I’ve had no issues using the phone while plugged in.)
I’ve had this on my FP5. I notice it mostly on a train; I was told it mostly has to do with “dirty” power causing the touchscreen to misbehave, but also that it’s fairly normal.
But I also did not have this problem with other phones (but also I did not have a USB-C charger back then, so maybe it’s the higher voltage?).
I see the same issue with my FP5 and did see something like that with FP2 already, too. The biggest impact here is the charger. I recently found one charger in our household that was particularly bad in that respect. With my default charger (one that Fairphone sold during Fairphone 1 times) I don’t have any issues. I didn’t hear of any issues so far from the FP6 user in my family.
The impression that it has to make with “dirty” (in terms of bad grounding, not well regulated etc.) power is probably a good picture of what’s going on there.
If you suspect dirty power (i.e. transients and stuff) for fencing in the problem try to use a power bank. Is the problem present with them as well?
Given that the one thing that is causing this issue is charging the phone while logging in, I guess ‘dirty power’ is a prime suspect.
However, I have several chargers all over the house - they all produce the same symptom. These same chargers do not cause similar problems with my other phone (Pixel 4a).
May I add that I make my electricity wipe its feet on entering the house. The main supply is clean. 
So, I narrow the problem down the way the phone reacts to having a USB-C plugged into the charging slot. Not a big problem - I disconnect the supply, login, and continue with using it.