No option for persistent agent in the application-aware settings

[ITP-Stan]

Case: #08205243

We are using the latest V13.1 VSA deployment.
We have a Windows server in DMZ that is not part of the AD domain.
I'm trying to do application aware backup of this VM.
At first I tried with username/password credentials in the form of .\Administrator or HOSTNAME\Administrator but no success.
After consulting with Veeam Intelligence and the helpcenter, it seems that NTLMv2 does not work anymore in the VSA version.
But a windows instance that is not part of the domain can't use Kerberos (yet).

So I choose a different route, persistent guest agent with the deployment kit.
After installing it, the "test credentials" in the job settings now give me the green checkmark.
But the job keeps failing to process that VM with guest-aware processing.
Checking the documentation again, there is mention of a checkbox to enable on the per-VM guest processing settings that should force the use of the persistent agent.
But this checkbox is missing when the job is a Proxmox backup job.

According to Veeam support the job should automatically choose persistent agent, without the need for a checkbox.
However in my case the job fails to do so and keeps trying with credentials.
According to the same support engineer, it's not a bug but due to " ... some conditions ...".
And the solution is to create a whole new job and map the chain, then do the same for all my copy jobs.
"it should work ..."

That is not very confident and I'm reluctant to create a new job and mess with my backup chains.
Anybody experience with this?