I am just a completely random MS employee, acting on my own behalf, who is browsing HN on my day off from work. I work in Xbox and don't work anywhere related to Excel or tech support. I just repro'd your issue in under 30 seconds in Excel 2016, and also submitted an issue internally on this. I hope it helps :)
http://i.imgur.com/N6X2Lj4.png
Also, I'd highly recommend using the builtin feedback menu, e.g. "send a frown" highlighting the issue. As well as making a post on https://excel.uservoice.com/. I know on my current and former teams, both of these(feedback & uservoice) get looked at quite often, even if there is only a few number of votes on them.
edit: +1 to https://answers.microsoft.com as mentioned by mherdeg