There was finally word from a Bing spokesperson saying that this was caused by Bing’s gradual move to encrypt their searches.
Our move to encrypt search by default as reported here — and which is currently ongoing — combined with how search traffic is being identified has likely contributed to the issue becoming more noticeable in recent weeks.
And here’s the thing, when it comes to Google, they remove the keyword data, but they’re still passing that traffic came from search. With Bing, they are removing the fact that the traffic came from organic search, in addition to removing keyword data.
More than likely, this could be just a consequence of Bing changing to HTTPS.