My next step is to make a separate Hosted Web App, this time via AppStudio (http://appstudio.windows.com) and see what code it generates (it also has option to generate ready-made compiled packages for uploading to Windows Store) and compare it (with free WinMerge tool) with the one generated by manifoldJS (http://manifoldjs.com) that I had used
not that the UWP Hosted Web App itself contains ANY code at all (it’s just a manifest file where you set for Start Page the https://… URL of the Bubble app in our case), but maybe manifoldJS spits out some wrong setting at the configuration files of the project or something like that
fingers crossed