We have a SharePoint 2019 on-prem environment with the CU from November 2019. We have created several of our own SPFx extensions that work good in 2019 and SPO. The same behavior happens as well on our SP2019 test environment which is identical to our production.
So what's going on
The strange thing is that if a user adds the extension to the site it will take the name of the last extension he/she added. Meaning if you added 3 apps all of them will take the same name as the last one you added. This will make it very difficult to see which apps you have on a site and if a update is available.