Hmmm, this might be related … https://support.fairphone.com/hc/en-us/articles/360048139032
"System update improvements
Your Fairphone will be better at managing storage partitions in the background, making system updates better-prepared for years to come.
- We retrofitted a new way of managing the system storage (dynamic partitions). This affects system partition and images only, it does not affect user data."
Assuming your bootloader is unlocked since you wanted to install TWRP:
https://support.fairphone.com/hc/en-us/articles/360048050332
According to the instructions there should be a prompt whether to erase or keep user data.
(Which is only meaningful when not unlocking or locking the bootloader which both force a factory reset for security reasons.)
Does this mean you have a backup of your data partition from before the update?
And a separate backup of the “Internal Storage” because TWRP backups don’t include it?