Here's some additional insight: the web page where the link exists (on the confirmation page) also wasn't present in my activity log:
But you really need to look at the browser Network log during these events to see if it's even attempting to send the activity. If it isn't, that's a very different problem to solve.
UPDATE (2/18/17): Marketo Support has been looking into this and they have been able to replicate this behavior on their end (where a small % of "clicks link" activity is not being tracked/captured):
At this point 3 out of 4 of my test records are showing all appropriate page views and clicks, however one has exhibited the same behavior as your test record. I am still working to isolate the issue and will update you when I have more information.
I'll provide an update as I learn more.
Sandy, sorry for going back in and revoking what I had originally thought as being the "correct" answer. Apparently Marketo support is looking at this thread as part of the diagnoses. Just yesterday, after I was able to prove once again that I was able to reproduce this (1 out of 7 occurrences), the latest reply from them is as follows:
This looks resolved. It was tested on your end a few times, one time there was an unexpected error but the other 8 times it did work. The error may be due to refreshing/clicks/something not related to the actual functionality in Marketo. Please let me know this happens again outside of repeated testing.
I'm quite shocked by this response, I must say.
I'd really like to be there for one of the repro cases, since we don't have a network log at the moment. Such a big difference between the Munchkin call completing w/200, being reset for load-shedding purposes (but still ostensibly completing), timing out, and not being sent at all.