Trying to add a feature service from standalone GIS server 10.7.1 to enterprise portal 10.9.1. There is a built-in user account created in server 10.7.1 to access the secured service.
In enterprise 10.9.1, the option Store credentials with service item. Do not prompt for authentication is missing. Can add the item with built-in user credentials, but asks to login every time the item is accessed. This deployment is not disconnected.
But, the store credential with service option is available when added to enterprise portal in 11.1 (this is a different deployment and network).
Looked at this post: Solved: When does the option "store credentials with item"... - Esri Community, but our deployments are not disconnected.
Also seen this article: Problem: Unable to Store Credentials with a Secured Service Item in Portal for ArcGIS However this is working fine in 11.1 without any SSL import so expect same in 10.9.1 as well.
The issue may still be SSL/TLS related, even if you never imported SSL certificates in 11.1. There are too many unknown variables for us to rule it out just yet.
A good test is to use the CheckUrl function in the ArcGIS Portal Directory:
If the response is anything other than a HTTP 200 status, it may indicate an issue with your 10.9.1 Portal's ability to verify the service's SSL/TLS certificate chain.
On a related note, it will be worth checking if you have restricted the portal's proxy capability
Run these tests and checks on both your 10.9.1 and 11.1 deployments to compare results.