Firstly, Tony, are you AJV who posted this issue to the ZWO forum in 2021-Jun-17? There doesn’t seem to be an answer from ZWO.
Becoming aware of this thread, I tested my own setup using ASIImg V1.7 (64bit) and both a ASI16200MM and a ASI174MM camera. For both cases the DATE-OBS header has the end time (or within a second or two of it) NOT the start time. This is despite the comment on the DATE-OBS entry which says “/ UTC start date of observation”.
I will also make my own posting on the ZWO forum about this. Unimpressed with ZWO on this.