OK, there was no chance for what you did to work. Private addresses do not offer a way for the plugin to be notified. That’s why callback addresses (commercial) are used instead. The plugin needs to generate these addresses as needed.
So we will first need to find out why addresses are not being generated (or not stored to the database). You are quite right, it is indeed a very strange situation. But this is the root cause of your issues.
Two things that come to mind:
1. I am not convinced that there are no errors in your logs. There should be at least some warnings, even if everything works as expected. More likely, there should be some error indicating either that the plugin could not contact the platform or that it cannot insert a new address to the database.
2. After clearing the tables, did you first visit the page with your [wallets_deposit] UI? This is what triggers a new address to be generated, when a deposit address does not exist.
Other than that, I cannot think of anything else right now. Once I think of something I will get back to you.