It seems like a long living bug, but just decided to investigate it deeper.
If I select metric, but keep English as my language, I always get miles.
If I want to see kilometers, I must change to another language, as English (UK) or Portuguese (my local language)
Is this a bug?
If I am not mistaken these “options” on the dashboard only apply to the ‘My Driving Stats’ section on that page.
See also, https://support.google.com/waze/answer/6153363?hl=en
If you change the region (language) at the bottom right to another language, this change is used everywhere else on waze.com. To be precise everywhere else that supports it, like ‘Live map’, ‘Major Events’, ‘About’, … (But not the forum). The metric/imperial system also comes with the region of choice. Another proof for this is that the date format changes from 15 jan (Dutch) to jan 15 (English) on the ‘Major Events’ page.
The only error, according to me, is that the region choice is only represented by languages.
It may be the case, but, if not a bug, it is a bad design (as pointed by you at the end). A quick search on the forum shows this is a usual source of confusion:
https://www.waze.com/forum/viewtopic.php?f=12&t=132012
https://www.waze.com/forum/viewtopic.php?f=12&t=91242
https://www.waze.com/forum/viewtopic.php?f=12&t=121980&p=986862
… and I could go on. Maybe a change here would be welcome. 
A helpful change could be to add a text like ‘switch stats to:’ before the world - metric fields. This way they wouldn’t have to change the design and it would result in less confusion.