Hi DozyWannabe,
The other reason is that the FD would have been taking it's information from the selected values, correct? According to the trace, none of the "Selected V/S" values during HN39's excerpt even come close to requiring a 15 degree NU climb.
I think with 2 or more ADRs disagreeing, the FDs bars will be removed, and all previous FD/AP modes are "forgotten".
When 2 or more ADRs agree, the FDs will reappear (because they were never turned off) and are "synched" to present aircraft attitude in HDG and VS (whatever the aircraft was doing at that moment). The FD bars will now command that VS achieved on re-engagement.
If the aircraft increased its ROD, then presumably the FD bars will command more nose up.
Probably an excellent reason to turn them off iaw the UAS procedure.