Yes, this has been reported as a bug in RWG and I see it is now in the "confirmed" category. For reference:
And if you want a work-around, remove (or comment out) these lines from the modlet's rwgmixer.xml file:
<!-- Adds residential to the list of possible wrappers for a Town for more variety. -->
<set xpath="/rwgmixer/township[@name='town']/property[@name='outskirt_district']/@value">rural,residential</set>
<!-- Changes the wrapper of a city to residential because there's already a lot of rural. -->
<set xpath="/rwgmixer/township[@name='city']/property[@name='outskirt_district']/@value">residential</set>
Thanks to
@Baradar for his testing on the matter.