Maybe this is a toolkit issue, but plasma-systemmonitor is what I noticed it in. For example, the hamburger menu above the left pane. The only way to close that menu is by clicking on an item, or clicking on the menu button again. Clicking elsewhere in the app doesn't close the menu, unlike apps like Elisa, with its hamburger menu.
can't reproduce here on master with the app hamburger menu, neither on comboboxes in edit mode
Looks like a duplicate of BUG 444808.
I'd suggest using native menus instead of QML ones, just like the panel and system tray do.
The particular issue this refers to has been resolved a long time ago, so I'm closing this.