Nearly all problems with sorting by capture date reported here are caused by the photos missing a capture date in their EXIF metadata. This can occur with very old digital cameras, scanned photos, editing software that doesn't preserve metadata, and social-media services that strip all metadata. "In september I changed the location of my catalog and image folder. ... a decent number of images have that September date as the "capture date" for files that are much older (and no original date exists in that field)." When a photo is missing capture date in its metadata, LR behaves very badly, using the photo file's date-modified (as shown in Finder / File Explorer) as the capture date displayed in the Metadata panel, under thumbnails, and used for sorting. When the file is modified for whatever reason (e.g. doing a LR Metadata > Save Metadata To File), LR will soon after start using the new data-modified as the capture date! This appear to be what's happened with the "September" photos. The only way to stop LR from doing this is to ensure that each photo has a consistent internal capture date in the catalog: 1. Select all the desired photos. 2. Do Metadata > Edit Capture Time. 3. Immediately click Change All. This does not, repeat DOES NOT, repeat DOES NOT, change the photos to have the same date. Rather it consistently records the date shown for each photo in the Metadata panel and under the thumbnail as the internal capture date in the catalog. I don't know if this is the procedure you used on the previous photos you mention. If after applying this procedure, you're still having issues, the only way to troubleshoot is to start with a specific photo.
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