Always On AG is Down when service is stopped
It could be the issue described in INF: AlwaysOn – The secondary database doesn’t come automatically when the primary instance of SQL Server goes down by Arvindh Kalidasan - Support Engineer, Microsoft GTSC.

In this blog we would discuss about behavior of AlwaysOn availability group where the secondary database doesn't come automatically when the primary instance goes down. The secondary database goes into Resolving state. On the failover cluster manager the resource appears in fail state.
[...] we found that if we stop SQL Service manually on primary replica, it fails over to second node only once. Any further attempts of stopping SQL Service (to test auto failover) would not cause failover.

The workaround posted there is:

[...] to have the value set to a higher number for &quot;Maximum Failures in the specified period&quot;.

Maximum Failures in the specified period: set to 60
Period (Hours): set to 1