But the connection generally is still active?
Sounds like some task falls into the sleep mode asa the screensaver gets activated. I can only remember specifically the wifi option to put it asleep when the phone is locked. But unless you are not using any kind of voip app for phoning I don’t see any relation.
Do you mean a special screensaver or simply the phone locking whilst the screen darkens?
I assume you have third party apps installed. Are these all the same as on your first FP2?
Have you had calls before you installed & configured your second FP2?
Unfortunately using the search function will list plenty of topics about the mic not working, also after updates.
I am afraid there is no final root to the cause. Reading through some topics and trial&error seems to be the only way left.
You may start off with this step.
If there would be a hardware problem I would expect the mic to never work. But the behaviour on you is slightly different.
Does it operate for longer when keeping the phone and screen unlocked for a longer (maximum) time?