User blog comment:Rappy 4187/Technical Update: July 27, 2016/@comment-25090105-20160730055253/@comment-126761-20160801150304

Overall, this is generally expected behaviour - the column is split into two columns that are the same width as the original. However, we do want to avoid it splitting a single module into two sections though.

Interestingly I only see that behaviour in Chrome, not Firefox. Could you send in a report to Special:Contact/bug, to help us look into this further? (And thanks for the suggestions Saftzie!)