I hope some of you must have noticed custom apps are uninstalled while upgrading Online tenants.
How can we keep them while upgrading?
As per Microsoft, If we upload extensions via extension management page then those apps won't be uninstalled while upgrading.
If we install extensions via Visual studio code then those apps will be uninstalled while upgrading.
So always keep it in mind to install/upload apps via extension management page and not via visual studio code.