iPhone X, iOS 16.1
Manual upload of photos and videos with .jpg / .mov conversion enabled in Dropbox.
Of 2,700 photos & videos uploaded, about 450 are accessible. The files that are accessible follow the expected naming convention of "Photo <mmm> <dd> <yyyy>, <hh> <mm> <ss> [AM/PM].jpg" based on when the photo was taken. The inaccessible files are named according to the time when they're uploaded. This would seem to indicate that the meta tags are corrupted or missing. File extensions don't seem to matter (.png, .jpg, and .mov) as a mix are accessible and inaccessible. Dropbox.com says the file can't be previewed and W10 Photo Viewer reports that "It looks like we don't support this file format." HEIF/HEVC formats are supported on my PC.
I'll try uploading in smaller batches to see if that resolves the mass corruption.