I have a theory for these problems (and some others)
Unlike most other phones on the market, Fairphones are meant to be disassembled easily. As this, contacts between components are “less solid” than in more integrated phones - where components are hard soldered together or using tight connectors.
In FP3 the motherboard is connected to the screen through a number of metal pads unto which springs press the counter-contacts. This, by design, is a setup that is vulnerable to having occasionally bad connections.
Over time the contacts will form a thin residue layer from dust that got into the device, moisture (not actual water but just the moisture that is normally in the air) and oxides that formed on the metal or on metal-metal interface. Some of this might already be there from manufacturing.
If then anything disturbs the contacts - for example the phone falling down, but just vibrations (getting a call), changes in temperature, or just normal handling, the contacts shift ever so slightly - from the spot where they had always been to a section of the pad that doesn’t conduct as well anymore.
Some of the time the spring force is enough to press the metal through the microscopic layer and create a good connection, but not always.
Some phone might have more residue, less spring force, or just bad luck, and you get outages like the screen or other periphery components failing.
This is the price to pay for a phone that can be disassembled without having to unsolder components.
But its also usually easily fixed, as the residue layer can be removed with the appropriate electronics cleaning materials.
In most cases, taking the components apart and scratching the contacts against each other slightly, so the pin heads scratch through the residue layer and get good contact again - or maybe cleaning the contacts with cleaning alcohol - is enough to fix the issue.
The contacts are definitely both the most likely and most easily self-fixable culprit for most electronics problem on the Fairphone. These issues now overwhelm support, and are likely the reason most manufacturers don’t make their phones repairable in the first place - soldered contacts don’t go bad. But at least in this case you usually CAN fix it yourself