First: This is a silly one. After updating to 3.4.11, please clear browser and website cache then edit or create new module page. By clearing cache our 75% users' issue was solved.
Second: This is template specific, if you or your developer ever override module then you will save the module but will not get any data in the module position (only module title is showing). What you need to do fix the issue is simple. Go to your website server: /your-website-directory/template-name/html then delete (take a backup before doing it) the folder: "mod_sppagebuilder", check your website, module is showing perfectly. Other 20% users' issue will be fixed in this stage.
Third: I don't know why they edit the module in this way. They open module in backend and frontend then edit that in frontend and save there, again come to backend (browser tab) and save also in backend without reloading the browser tab so they get empty module because without reloading browser tab their edited content don't come in backend on the fly. Our recommendation is that don't use two editing options. Use only one, either backend or frontend. Other 5% users' issue will be fixed in this stage.
We covered 100% solution.


Note: Module duplicate is not working. Yes, that should not work in new module system. We can't give support for that option but there is another way to do that. Use "Export" and "Import" option to duplicate module.
Warning: If you ever saved any module before clearing cache, that is not recoverable. So you need to re-create that module with latest version Page Builder.
Thanks for reading.