Currently, SQL Monitor DB is on the SQL Server cluster, in the availability group.

When I'm doing the application update, the update process reads old connection string (<add name="DataConnectionString" connectionString="Data Source=ops-sqlmonitor-ag.ad.npspot.com,6803;Initial Catalog=OPS-SQLMonitor;User ID=RedGateSQLMonitor;Password=***************; Max Pool Size=500;Connect Timeout=5;Encrypt=False;Trust Server Certificate=True;Packet Size=4096;MultiSubnetFailover=true"/>) from the config file, but when it writes it back, it has removed the ";MultiSubnetFailover=true" -part.

This means the connection to the DB may not work because DNS returns multiple IP addresses, and the first IP address isn't always the primary node.