Well..... after rebooting and setting Cortana to never run in the background, I left the phone overnight on WiFi, with a strong ATT signal, BT enabled but not connected, NFC off, DTTW and DTTS both enabled. The standby drain the next morning was 0.4%/hour. That is outstanding and only about 0.1%/hour worse than my Nexus 5X on Android 7.1 beta (which I consider my benchmark).
So it would seem that my issue yesterday was not build 14965 per se, but more that some process like Cortana ran amok and drained the battery. That needs to be dealt with.
I have temporarily set Cortana to never run in the background and not to respond to a long press of the search icon while locked. Lets see if the issue recurs.
For me at least, these background process issues are VERY long standing in both WP and Android. Android seems to have cleaned up its act recently and WP is also making progress (0.4%/hour is VERY good). As I recall, the production build on my 950 could do about 1%/hour in standby under similar conditions. I started my WP journey on an 820 running WP 8.1 but gave up on it when I found that the battery drain in standby was very erratic. I could never rely on having usable battery left if I left the phone unattended for a day.
Paul