I use mobx-devtools as an extension in Google Chrome with MobX 6.6.2.
mobx-devtools only starts working and tracking changes when MobX is initialized immediately after the page has loaded. And even in that case, it skips changes until the message Connecting... disappears.
I'm not sure what API mobx-devtools uses to track changes, but it's possible to track changes using the window.__mobxGlobals.spyListeners array. window__mobxGlobals is available immediately, and I have to use it when "mob-devtools" shows the message Connecting... or simply doesn't work.