Some dashboard subscription disappear

I have a dashboard with email subscription at specific hours,
one subscription is at 5:00, the other is at 8:00, both have the same 9 receivers.
The next day receivers ramdomly disappear becoming 4 for task at 5:00 and 5 for task at 8:00
the issue in persistent even upgrading matabase or rebooting service

Metabase version installed now is the last available, 0.50.9

Are you sure these users aren't unsubscribing form the subscription?