Figured it out myself: The instructions say to insert the stainless steel insert into the seal, install the seal in the bottom of the tub, and then assemble the hub to the shaft.
But the insert does not fit tight to the hub, and water leaks past it!
To fix the new leak, the stainless steel insert must be glued to the hub.
I used Locktite sleeve retaining compound, and so far, it is not leaking.
The insert must be centered on the hub, or it will wear the seal rapidly.
I put on the Locktite on both the hub and insert, and then slid the assembly onto the shaft, part way into the seal. This centers the insert on the hub.

I must say, I'm not particularly impressed with repair parts kits and instructions with built-in leaks!