Hi again @inscriptionweb,
Thanks for confirming that the removed subscribers are completely gone from the database.
I do see that you have a third party database cleaning plugin, Advanced Database Cleaner:
https://en-ca.www.ads-software.com/plugins/advanced-database-cleaner/
As we are not the developers of that plugin, we cannot speak to its functionality or how it might interact with MailPoet; however, it does seem like the most likely candidate here for having deleted items from the database. I see you also have a downgrade and a rollback plugin as well; both of these could also have had an effect on the database directly and altered tables unexpectedly.
As we have received no other reports of data being deleted by upgrading MailPoet, this does not seem to be a issue with MailPoet itself.
All MailPoet data is stored only on your local site. We do not maintain a centralized backup of your data for privacy reasons. So, if these subscribers have been actually deleted from the database, the only option to restore them is to restore the database from a backup. If you have a copy of your list from a previous backup, you can also import that to your subscriber list to restore it.