Outside temp 10 degrees and heater is blowing cold air.

Make sure there's coolant in both systems. A regular OBD2 reader should be able to read the engine coolant temperature - check to see if that warms up when the ICE runs. Leave it unplugged so you can use the CHG mode to make sure the ICE runs.

This should help to isolate your problem, a bit. At least it will eliminate whether the engine has coolant, and that there's heat in the coolant.

 

I'm going to assume that you were running in EV mode, if the air suddenly went cold - you may have went from cabin (recirculate) to outside air. If your ICE motor was running, the problem would be in your HVAC blend doors or coolant circulation system.
I run my HVAC system on manual most of the time. That prevents the robots from doing weird things I don't like...

 

you won't get heat from the heat pump below 14f or so. the early heat might have been stored from a previous drive? idk

 

I would expect the car to switch to HV mode if asking for cabin heat when too cold for the heat pump.

 

A couple of settings to try that may help at least some with getting the heat to stay warmer with these temps many of us are seeing this week.

switch from ECO or NORMAL modes into Power Mode (HAVC systems cycle on/off less often)
Use the dedicated windshield defroster mode ( to keep the gas engine running longer / cycling off less often ).
Turn the heater temp. up higher ( tells the computer to keep the coolant temp higher by cycling the engine ON more often ).
Check the owners manual for tips on how to be more efficient in freezing temps. And be less concerned with efficiency during extreme low temps.
hopefully there should be something in the manual explaining how to active the battery pack heat systems ??. like the Pre Conditioning feature (if Gen 5 still offers that feature).

(if Traction Battery Heater or a similar Gen 5 equivalent feature is available ) Leave the Prime plugged in to take advantage of it. ( max number of days in Gen 4 is 3 days ) check the owners manual for Gen 5 recommendations for Traction Pack in extreme cold conditions.

( Setup charge time feature to charge the Prime a few days from when plugging it in ) to allow the Traction Pack heaters to work without charging the Traction Pack as soon as plugged in.
Charge mode will not work above 80 % SOC so if you want to use it to keep the engine running keep the SOC around 50% and that should give you maybe 20 minutes of engine run time before it switches back into EV mode. After charge mode automatically shuts off, use any of the other tips to help keeping the coolant temps up there.

grille block ( pipe insulation trimmed to fit grille openings works nicely when temps are below freezing, even with the "dynamic grille shutters" - just remember to pull them out when temps go above freezing).

maybe @KMO and // or @Hammersmith have more Gen 5 tips and tricks for extreme cold operation specific to Gen 5 .