I think I understand what you are describing.
The Fairphone 3/3+ is an A/B device, it has two slots for the OS (A and B). While you are using one slot, OTA updating the OS is done by installing the updated OS on the other slot in the background, so you can use the phone while this is done, and then there’s only a short reboot to switch the OS slot to the updated one. User data (= the data partition) is shared (= not in two slots) and stays the same.
One of your OS slots seems to not like what it sees regarding the data partition when booting after the update, getting you to a screen with the options to “try again”(booting) or a factory reset.
You choose the factory reset … and it gets the OS to boot successfully on this slot, so you are using this slot then.
The next OTA update then installs to the other slot which doesn’t have the problem … so no problem.
The next OTA update after that then installs to the bad slot again … problem.
And so on.
The solution should be to get both slots into the same working state again.
If you readily do factory resets to work around the problem, you seem to be prepared for wiping your data, which is a good prerequisite to tackle this head-on.
I see two obvious options.
You could install /e/OS again. The manual install explicitly installs to both slots (for FP3 you can choose the dev or stable release channel). The Easy installer should install to both slots, too (but only stable releases).
You can clearly see which release channel you are currently on in Settings- About phone - Android version (tap on it) - /e/ version.
Alternatively, you could reinstall Fairphone OS to get the phone back into a well defined state with every possible partition set up how it should be.
As the /e/OS install doesn’t cover every partition there is in the phone, this would be an even more re-set starting point to then again install /e/OS.
