Problem: Some images have no creation date. Could be copies of old photos, old photo scans, incorrect metadata, etc..
I'm consolidating photo libraries from the past 15 years or so and ran into an issue with some older files that had no capture date metadata. Lightroom is smart enough to look at the date modified so when I go to sort images by years everything looks good. But when I go to export images / rename and export, the date modified on the file gets changed to that day. And since no capture metadata exists, it sets the new date to the most recent file creation/date modified. Additionally there's no way to sort in lightroom by images that have no creation date, or by date modified. So when I would do a batch export with new filenames, the date/time info for about 1/5 of the images was getting lost.
The fix: Rather than going through and individually fixing each photo or even doing it by groups, you can select all of your images go to "Metadata>Edit capture time" then select "Shift by set number of hours" and add +1. Lightroom will then write new metadata to ALL of the photos, including the photos that previously had none, but using the info from the "date modified" file info. You can then repeat the process and shift the number of hours by -1, which would reset all of the times back to what they originally were. You should now have accurate capture date/time metadata for all of your photos to do with as you will.
Hope that's helpful!
As you've discovered, LR doesn't handle very gracefully files that are missing capture dates in their metadata.
In your workaround, it isn't necessary to shift by +1 hours and then -1 hours -- just select all the photos, do Metadata > Edit Capture Time, select Adjust To A Specified Date And Time, and click Change All. This will have the same effect as your workaround, but is faster.
Please add your vote and opinion to this bug report in the official Adobe feedback forum: Lightroom: Still inconsistent capture date/time for photos and videos