Uninstalling an app in cloud resulting in lost of customfields values

I would like to ask for your help regarding an issue we have in cloud when trying to perform updates to our apps.

Recently we’ve migrated to cloud (sandbox for now) and we started developing the apps. Some of our apps are creating customfield types.

The problem is that the customfields created using our customfields types are losing their values when uninstalling and installing back in production environment.

Can someone tell me what is the procedure to follow when trying to update an app? In order to see the logs of our forge app (using tunnel), we need to install the app in the development otherwise the logs are not present.

How can another developer can perform modifications /fixes to the app without losing the customfields values?

Thank you & regards,
Ciprian Damian