Availability group replica was lagging behind the primary and was not catching up. One of the secondary replica was set to "Synchronous mode" with auto failover and that replica was also lagging behind the primary. There
were nothing unusual reported in the SQL Server error logs nor the Windows log.
Environement:
3 Nodes in Primary data centre
one Primary, one replica with Synch mode with auto failover and other in Asynch mode.
3 Nodes in DR site with Distributed AG.
Kiran