Because Google have finally seen the light in recent Android and made this feature less easily available as it is inviting trouble.
Incorporating the SD card into Internal Storage comes from a time when phones had minimal storage and users as well as vendors were desperate.
Technically it wasn’t a good idea back then, and it isn’t now.
If you really want to do it there’s still a way, but be prepared for Google to pull the plug eventually in the future …
There’s an official partnership in place with /e/ …
What if it is software?
I know a factory reset isn’t popular, but it tends to help with software trouble.
Should the fault somehow lie with the OS itself (which doesn’t get touched by an Android “factory reset”), the OS can be reinstalled, too … https://support.fairphone.com/hc/en-us/articles/360043337512.
And then there’s still Fairphone support you could contact …https://support.fairphone.com/hc/en-us.
It’s not impossible that Android 10 would fix something, but are you assuming everybody would have the same issues you do? That’s not the case.
If you really want to make Fairphone aware, you will need to contact support.