Let me start with the specifics:
Mac Big Sur 11.6, Reolink Client 8.3.1/8.3.2, Camera's 820A, 822A, 520A all with latest software and all with at least 30Gb SD sticks.

Problem:
When using FTP to a WD Cloud server the format of the MP4 file will not view in at least 3 viewers that had been used in the past under version Reolink Client 8.2.6!File on SD stick views fine within Reolink client. Error code from video viewer reads that the MP4 is NOT in a MP4 format. Videos downloaded from the SD sticks are viewable.