Version 2.3.1440.48 (Stable channel) (64-bit)
OS Windows 10
It's happened with every extension I've tested that opens options as its own local page under chrome-extension://. That is, any extension which has the manifest option "options_ui"->"open_in_tab" set to true. If the extension instead opens options as a popup on the extension page, right-clicking and selecting inspect will successfully open dev tools without issue. Here's a list of extensions I tested:
It's set to notification only and it's hard to reproduce. First of all mentions itself are rare, because you normally get direct replies to your messages in topics you have posted in, which don't count as mentions. In case someone mentions you outside of this context and you don't get a notification and you don't read the topic by chance, you cannot know.
When we tested this at the start of the thread, I first didn't get mention notifications and in the end I did get one. It was kinda random.