My Xiaomi Mi 5s cannot backup Gallery to Mi Cloud for couple of weeks either!
It backs up some Gallery items, but leaves the majority.
It is using 10,21GB out of 25,00GB, waiting for 450 to be synced.
Location: Türkiye
Android version: 7.0 NRD90M
MUIU version: MIUI Global 9.6 | Stable 9.6.1.0(NAGMIFD)