Chrome version 59.0.3071.115 (Official Build) (64-Bit) seems to “suddenly” have changed how it treats popups in that it opens them as a new tab. This is at least what we observed with the update on 2017-06-26 to 27 to version 59.0.3071.115.
Even setting the popup switch to “allowed” does not appear to change this behaviour: How do we get the popup behaviour back?