update: I've been playing around with the problem some more and I've discovered the following:
Trackable Issues
The problem exists when the timezone of photos from select sources are adjusted.
The problem exists when the time of photos from select sources are adjusted.
Semantics: by "share/export", what is meant is the following. "Share" refers to the built-in share function. "Export" refers to the ability to drag the photo onto the desktop to treat it as a file.
The test performed was this:
Part I
A1. Import a selection of photos. (Sony Cameras, Nikon Cameras, Canon Camera, DJI Camera, iPhone Cameras, Android Cameras)
A2. Check to see if I can share/export all of these files that have not received any modifications.
Results
All Sources: Success
Part II
B1. Adjust the timezone for a single photo taken from each source.
B2. Check to see if I can share/export these modified files.
Results
Nikon A: Success
Nikon B: Failure (Model: Z-30)
Canon: Success
Panasonic: Failure (Model: DC-G9)
DJI: Success
Sony A: Success
Sony B: Failure (Model: Rx100m3)
All phones (both android and iPhone): Success
Part III
C1. Adjust the time on a different selection of photos from each source, without adjusting the timezone. (adjusting the the time by five minutes.)
C2. Check to see if I can share/export these modified files.
Results
Nikon A: Success
Nikon B: Failure (Model: Z-30)
Canon: Success
Panasonic: Failure (Model: DC-G9)
DJI: Success
Sony A: Success
Sony B: Failure (Model: Rx100m3)
All phones (both android and iPhone): Success
Observation
Photos taken by certain cameras, when the time and/or timezone is adjusted, causes the Photos App to no longer be able to share/export said photos. The ability for these photos to be uploaded to iCloud is not affect. Results are the same across devices once the files are in iCloud. (share/export functionality is the same on iPhone as it is on the Mac where the files were uploaded. If I cannot share/export on the Mac, then I am unable to share/export on iPhone. If I am able on the Mac, then I am able on iPhone.)
Kooky Workaround
Oddly enough, if any of these modified images are "cropped" without actually doing anything to them, then the program is thereafter able to recognize that the photo does exists, and is only then able to share/export it.
I spent a few hours on this; I hope this can help lead to a solution in the chambers of the engineering department. I would reaaaaaaaaally like to see this issue get fixed... Thanks!
-Affected Consumer