It doesnt sound like you set-up your Tx as 3.8V is the default for the voltage alert.

Depending on what you set your ESC and feel comfortable for your lipo cut-off, you could change it to 3.2 - 3.4V or whatever you want. Or just clear it with the cancel button. Or you could just ignore it.
3.8V remaining is quite a bit of runtime left IMO and really, the Tx is reading just receiver voltage, not pack voltage. Measure what voltage your lipo has when this alarm comes on, it would interesting to see if any difference is seen.

Not sure what GBA is saying 10 min is the norm for a 5000Mah lipo or power is reduced in half as I find both those statements untrue. No reason you shouldn't be able to get 20-30 min runtime with a 5000Mah lipo unless we're talking flat out full throttle or you're geared way off. As far as power goes, it does reduce nearing cut-off but 1/2? Are we talking Nihm here?