Since its last update, Lightroom Classic (Release 10.1.1 for Windows) has been importing erratically. When I import Canon CR3 raw files and try to rename them sequentially by capture time, the renaming is sometimes wrong. The renaming scheme I use is YYYYMMDD_NNN where a three digit serial number NNN is added to the date. So, the first photo taken on 31 January 2021 would be 20210131_001.CR3 then the second photo taken would be 20210131_002.CR3 and so on. Sometimes LR does this correctly, as it always did before, but now sometimes the serial number is added out of order. The earliest photo might correctly be _001 but the photo taken next will be _060 instead of _002 and the third _021 or whatever. So, if I then sort after import by file name, the photos are numerically in order but they are not also one after the other by capture time as they would be had the renaming/numbering proceeded correctly. If I sort by capture date, the photos are in the right order by time of shooting (LR can apparently read the capture date), but the filenames are not consecutive. In Windows Explorer, I can sort properly by "Date Taken", the Windows equivalent of LR's Capture time, so the issue is not due to some problem with the metadata or file attributes. This has happened with photos from two different Canon models, so it's not likely a camera file handling issue either. I haven't changed any LR settings. This started after the last update and I assume it is a bug. I hope someone from Adobe will pick this up and comment.
The problem is likely to do with the fact that Classic's import is done in parallel, rather than sequentially, i.e. several files are being processed at the same time using multiple threads. That means that they may not necessarily all complete in the same order as they started, which can result in the "renaming with a sequence number" being out of order. If you really need a sequence number, the simplest way to avoid this issue is to delay the rename until after the import batch has completed.