The issue is that Tower refresh failed even though credentials were successfully validated. The Tower refresh failed with this message :
MIQ(ManageIQ::Providers::AnsibleTower::AutomationManager::Refresher#refresh) EMS: [Tower Automation Manager], id: [99000000000005] Unable to perform refresh for the following targets:
- undefined method `vault_password' for #<AnsibleTowerClient::Credential::Inputs:0x000055eea78a7578> Method:[block (2 levels) in <class:LogProxy>]"
This issue doesn't occur if there is a Vault credential on the Tower setup.
I verified the issue on a 5.11.2.2 appliance managing Tower 3.6