To troubleshoot this, you will need to turn the debug logs on. However, my personal preference for simple rules like this, is to use Basic Rules app. That is not to say, Rule Machine shouldn't be used. Please turn on logging and also post the settings page of your rule.
Yeah, that's definitely a problem. See if hitting "Done" or "Update Rule" (both do basically the same, except "Done" also closes the page and takes you back to the list or wherever you were before) helps. This should (re)subscribe to the garage door event. If not...maybe a bug? You could try doing the same with a new rule just to be sure. I haven't created any 5.0 rules with garage doors or contact sensors (not sure what capability you used), but maybe it could be reliably reproduced.
It should. Unfortunately, I am not able to replicate the problem. I created an identical rule and my hub subscribed correctly to the garage door event. Please send me your hub's ID in PM so we can take a look at your hub.
I am if I choose "Garage door open" (or seemingly anything--"changed" also failed) as the trigger event with the "Garage Door" capability and this virtual device I used for testing: