LR has long had issues sorting some kinds of photos by capture date, though one of the issues was finally fixed after many years. To sort this out, please answer these initial questions:
1. Do Help > System Info and report the exact version number.
2. Approximately when were the missorted scans imported into your catalog?
3. Were any of the photos synced from LR Mobile, the new LR CC, or LR Web?
1) Lightroom Classic version: 7.1 [ 1148620 ]
Operating system: Mac OS 10
Version: 10.12.6 [16G1114]
2) Complicated, because I was consolidating libraries from multiple sources. These came from iPhoto, and were imported via the LR Plug-in this month (January 2018).
OK, that helps.
LR long had inconsistencies in the way it treated imported photos missing capture dates stored in their metadata (e.g. scans). LR maintains a photo's capture date in multiple places in its catalog but different parts of LR failed to keep those multiple instances in sync. This resulted in problems with sorting, filtering, and renaming by capture date.
To see if that's affecting your scans, do View > View Options to ensure the capture date is showing under the thumbnails. Select a scan that may not be sorting properly and compare the date shown under the thumbnail with that shown in the Metadata panel:
If they don't match, then that photo has been struck by the old bug.
You can fix the inconsistency by selected a photo, doing Metadata > Edit Capture Time, and immediately clicking OK. This will make the date shown under the thumbnail match that shown in the Metadata panel. You can fix an entire batch of photos by doing Metadata > Edit Capture Time followed by Change All (this does not change all of the photos to have the same date but rather is the same as doing the procedure on each photo individually).
Since changes to capture date are no undoable, make sure you save a backup of your catalog before you start!
I should have mentioned that I had already turned on the Capture Date/Time for both Compact and Expanded Thumbnails and compared them (I found that old bug in a forum search).
Bottom Line, they match
However, , I tried your suggestion of Metadata > Edit Capture Time and immediately clicking "Change" (not OK ) and it worked!(and, yes, I backed up my catalog first). I must have backed up the catalog 100 times during my portfolio consolidation, and will have to clear out old backups at some point. Too paranoid to do it now.
So somewhere there was a different Capture Time stored.
I will eventually get used to these little quirks, I am relatively new to LR (although been using other Adobe Apps for years), because I hopelessly clung to Aperture for so long.
However, , I tried your suggestion of Metadata > Edit Capture Time and immediately clicking "Change" (not OK ) and it worked!(and, yes, I backed up my catalog first). ... So somewhere there was a different Capture Time stored.
Going forward, you shouldn't have to do this with newly imported scans. I tried to test the bug fix in LR 7.1 thoroughly, and it seems to (finally) be correct, knock on wood.
That, and I am doing all my importing to LR directly.