why can I not add 4k videos to shared album using Photos Version 5.0 (161.0.120) on Catalina 10.15.7?

My family and I took a bunch of videos and photos during our vacation. We created a shared album and tried to upload all of the photos and videos to the shared album (from within Photos on OSX).

I kept getting "export failed" errors. I finally narrowed it down to any video I made with my iphone 12, at a resolution of 2160 × 3840, HEVC, and 29.98 FPS.

The file sizes are smaller than some of the videos from the iPhone Xs, at 1920 × 1080, HEVC, and 60 FPS.

Why can't I add these videos to my shared album using the mac ?

As an aside, if I select the video on the iphone itself, and then tell it to add to shared album, it does with no problem.

thank you