Due to a bug that was discovered and fixed (EventLogging subscriber module in ready state but not sending tracked events T175918) by the Readers Web team on Sep 28, 2017, we'll need to annotate our mobile web dashboard as this shows a sharp uptick in events since the bug fix was deployed. It also seems to account for the decrease in logged events since March 2017.
Unfortunately, the Discovery department uses mw.track in quite a few (if not all) of our frontend A/B tests. Two of the more recent tests were the explore similar links (T149809) and language links (T175647) tests that didn't get a lot of events. This mw.track bug probably caused the decreased logging of the usage of the language links and the subsequent stoppage of testing.