To start, after a move I noticed my reverb was not working. I applied pressure to the upper RCA connections and it worked again. The next day it was out again. I just took the tank out to check and clean connections. They are okay. Maybe the RCA cable or pre-amp tube? I will see if this works here shortly..

However, when looking at the power tubes, I notice that 3 are GT-6L6B and one is a Fender 5881.

Is this okay? factory?

I expected to see 4 x GT-6L6B


1990 Fender Twin Reverb Re-Issue
Reverb cutting out could be a bunch of things, often it's the jack itself. See if you can get in there with a cotton swab and some contact cleaner to clean it out. Reverb cables and tubes are also worth a look, as you suspected.

Mismatched power tubes aren't optimal. It's unlikely they'll cause any damage, but to run the amp most efficiently you should have a matched set. I wouldn't worry about it too much, get a full set when you retube. If they're mismatched the power tubes will probably start making noise earlier than usual.
It is possible that the 5881 has the same bias rating as the other tubes (a matched set electronically, not visually) which would be totally fine. I had 2 gt6l6 rated at 7 and 2 gt6l6 rated at 2 (groove tubes approximate their bias rating with a number for easier matching)in my 65 twin ri, and after about three years of playing at a moderate volume my filter caps melted. My amp tech said fender ri amps often have cheap filter caps anyway so now I have some nice meaty ones in there.