Completed Catalina upgrade. Photos showed many referenced photo files that it had previously been linked to as not being found. Repaired library. Same problem. Created new library from scratch. Same problem. Initially the photos are shown as being referenced, but the next time I open app, some are not.
I'm running Photos on an iMac Pro purchased earlier this year. 64GB, 3 Ghz 10 core Intel Xeon, 1.39 of 2 TB available. Approximately 38,000 photos, 500 videos.