AH HA!!!! Think I found it, files were shot with 2 bodies, but filering by serial number shows only one is miss sorted. The one which had the wrong time zone set and for which I mass changed the capture time in LR after import. And the files are only out of order withing burst which share the same 1 second time stamp. So however it sorted them by capture time correctly when they had the same one second time stamp before was broken when the capture time is bulk edited to move them forward an hour. Checking the "Date Created" field I see that the ones whose time was altered lost the fractional second that the images from the other body have in that field. But I've now figured out why the selection cause the sort issue, the sort issue was partular to the set of images being filtered, which seems to actually be releated to photos grouped by stack that had their capture time edited. So I guess I'll delete this and then refile it when I better characterize why the group by stack is messing up sorting, because unstacking fixes it But it *also* breaks sort by file name!!!!!
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