I’ve had the same problem for quite a while and tried all the tips mentioned above and probably some more. Nothing really helped (at least not for long).
So I contacted the support about the problem and actually stayed in touch for a long while mailing back and forth about progress or the lack thereof.
When it was clear, that none of the “cleaning” worked, I was asked to send my phone in.
Now I just got of the phone with the support and I was informed that my new (exchanged) phone was on its way to me.
So apart from having to use a spare phone while mine was on the way, fairphone was very helpful and exchanged my product with no problems because of the state it was in (concerning the display problems).
I’m just writing this to let you know that
- There might not be a solution for now, if you do have this problem
- You should contact support and find out, if they will exchange your phone.
In short: Being in touch with the support worked fine for me.