Cannot add internally hosted Enterprise layers to Map WebPart in Maps for Sharepoint

[Jay_Gregory]

I have installed ArcGIS Maps for Sharepoint on a Sharepoint 2019 Foundation/Server farm, and configured it with a site collection to connect to our ArcGIS 10.8.1 Enterprise.  For the most part things work as expected except for one key component: I cannot add our own Enterprise layers (layers hosted on our ArcGIS Server) to the Map web part.  I am able to search our organization for layers and show results, but once I click the Add button, it just spins and the layer never adds.  However, I can add layers that are hosted on ArcGIS Online (such as Living Atlas or other layers pointing to FeatureServices on a publicly accessible ArcGIS Server).  

This would indicate that possibly our Sharepoint servers do not have correct DNS entries for our own Enterprise, but I have confirmed that they do have the correct entries.  Furthermore, I cannot find anything in the logs.  Has anyone seen this / have any suggestions?

[Jay_Gregory]

To further complicated things, I am also able to add Hosted Feature Layers (i.e. those in DataStore) stored on our Enterprise, as well as Map Service Layers and Web Maps stored in Enterprise.  However, FeatureLayers still won't add.  That is, I can add a MapServiceLayer directly, but am unable to add one of its underlying FeatureLayers.