Not sure, but this happened to my wife and I on award GF tickets SFO-LHR. Outbound was cancelled due to MX, and they cancelled return as well. They made no attempt to rebook. I had to hussle and find replacement flights on my own, then call UA 1K to have them rebook me.
Maybe UA just cancels award travel if there are any flight cancellations and makes to attempt to reschedule. Not sure.