Could you please verify what account is used to activate the extension. Esri vector basemaps are supported with ArcGIS accounts, not supported with complimentary or ArcGIS Maps for Adobe Plus accounts. Here is a link to the functionality matrix. Please share what account type was used, this will help inform next steps. Thank you for your post.
This appears unexpected given your setup is correct. We aren't seeing similar reports like this. Any chance you can report this as a case to Esri technical support. I think a screenshare & additional workflow info might be useful to determine the issue and a cause. Thanks in advance for your help.