It appears that in Swift 1.19.0, the primary page set on the product group is not being honored the same on products with variants compared to standalone products (products without variants).
I was testing to see if 1.19.0 had cleared up the issue reported here: http://doc.dynamicweb.com/forum?PID=1605&ThreadID=85177 and found the following issue instead: https://www.screencast.com/t/AqwrVPnGI. The solution shown is on DW 9.15.3 and Swift 1.19.0.