This is an annoying, sporadic problem, but I think I've finally figured it out:
When you use the "Edit Capture Time..." option to shift the timestamp of a photo (e.g. -5 hours),
If the photo's original timestamp has a 00 seconds component (e.g. "2016:08:23 22:23:00")
Then upon Export, Lightroom will truncate the seconds component of the timestamp (e.g. "2016:08:23 17:23")
Photos.app does not seem to be able to parse this truncated timestamp, and reverts to using the photo's original, unshifted timestamp.
When you use the "Edit Capture Time..." option to shift the timestamp of a photo (e.g. -5 hours),
If the photo's original timestamp has a 00 seconds component (e.g. "2016:08:23 22:23:00")
Then upon Export, Lightroom will truncate the seconds component of the timestamp (e.g. "2016:08:23 17:23")
Photos.app does not seem to be able to parse this truncated timestamp, and reverts to using the photo's original, unshifted timestamp.