Well, it's not just instrumentation that the HDG runs.
In addition to the Capt's Flt Inst Bus, it also runs
L AC Tfr Bus
R AC Tfr Bus
Stby AC
Stby DC
Batt Bus
Hot Batt Bus
So if, as you say, it packs up on landing, you would be left with only the Hot Batt. Hardly ideal for taxying and unlike Boeing's usual logical approach.
I guess you've seen this in the Sim rather than the plane, so my guess would be that this is a Sim quirk; I'll check in the Sim next time I'm in.