This happens when you do not sign the app using the same keystore. Generally, the App Inventor server stores your keystore in your account, so either you are switching accounts when building or you are switching between servers. You should not be getting this error when building on the same account on the same server unless you've explicitly replaced or deleted your keystore.
I know its hard to believe me, but see what I found,
If you set the webview to display and html page and build and install the app, and then try to change the html page and re-install the app, it throws a conflicting package error and will not update the app, if you put name of the html page in the design section:
If you use simple html file containing only basic syntax of html it works fine, but it shows the problem I mentioned on updating the app when an html file like this is used: Recommended HTML File to be used to testing
Yes indeed, difficult to understand and believe. To be clear, I doubt it.
Upload both APKs to Google Drive and post the links. I would like to check it myself.