Hi Gery! To answer your question, it is a combination of the two.

The plugin is different from some other plugins because it uses a separate engine application (bbw4) to perform many of the functions related to rendering. Some functions like opening certain dialogs require the focus to shift from the plugin (and Cubase) to bbw4. Cubase has a unique characteristic where it hides open plugins when it is not the focused application. You could test this by running Cubase with a plugin open, and then clicking anywhere off Cubase (on the desktop, or in a File Explorer window) and notice that the plugin will disappear until you click back onto Cubase. So when you open the Style Picker (or any dialog) in the plugin, the focus moves to bbw4 and Cubase hides the plugin window until you close the Style Picker.

Cubase is the only DAW where this is a problem, and it is quite strange to have to click on the DAW to make your plugin reappear, so I will try to look for a solution and hopefully implement something to prevent this behavior.