I just checked, builds are still signed with test keys and that’s unlikely to change.
Just imagine the support nightmare it would create if Fairphone suddenly started to sign their releases with a different key. I’m not even sure it’s possible to update the built-in root of trust, no idea 
If that’s the case, they’d have to switch to a user-settable root of trust, like some of the custom ROMs are using, which presents the user with a nice 
yellow warning screen on every boot.
FP support probably won’t be all that helpful here, you’d have to be lucky to get redirected to one of the developers. Either way, this issue is unlikely to get resolved 