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Triple-check that the camera clocks were set to the same time.
The easiest way to do that is to display the capture date under the thumbnails in Library grid mode. In Library, do View > View Options and set the bottom or top label of Compact Cell Extras to be Capture Date/Time. Then set View > Grid View Style > Compact Cells.
Then do View > Sort > Capture Time. Look carefully at the date/times under the thumbnails, and pay attention to AM/PM (if you're in a country with 12-hour time formats). Are the date/times in strictly increasing order?
If they're not in strictly increasing order, please post a full-resolution screen shot of your entire LR window showing the problem.
Yup, that was it. one Camera was off by exactingly 1 hour. I feel silly
It's something that's very easy to overlook, especially with the small fonts that LR uses. Glad you got it resolved.