Thanks for the information, @Acorn.

I checked the info.xml file in each of my WDP folders.

For v17, v16, and v15, each shows exactly the same thing:
<?xml version="1.0" encoding="UTF-8"?>
-<root>
<registered>true</registered>
<pugdate>2021-03-03</pugdate>
<versionValid>true</versionValid>
</root>

Interestingly, for v12, it shows a different pugdate:
<?xml version="1.0" encoding="UTF-8"?>
-<root>
<registered>true</registered>
<pugdate>2018-09-04</pugdate>
<versionValid>true</versionValid>
</root>

I can only assume that browj2 is right. Magix “thinks” I purchased two different copies of WDP… one when v12 was current, and the second when v15 was current. The latter is the one that I am currently using, and my fallback version should be determined by the most recent time I renewed it… last September = v16.
When I contacted Magix last week, they apparently saw the entry for the first purchase, and based on that, told me my fallback was v12.
The logic of their error is becoming clear… but the logic of the earlier error, of assigning me two product registrations, is still a mystery.
I plan to send them another message to try to get to the bottom of the issue.
Thanks again.