Aha - thank you, Susanna!
Searching through the Moodle PHP code, I see that the "Viewing the __" string is declared in the following Moodle file:
It is called "viewingactivity" and it is used in the "get_details()" method of the "completion_criteria_activity" object, which is in the following file:
As you can see from the PHP code, the only completion criteria that get displayed are for (1) viewing, or (2) achieving a certain grade. Any other completion criteria are ignored in this report. Therefore, I believe that this issue affects not only the HotPot module, but any module that uses criteria other than viewing or achieving a certain grade.
This seems less than satisfactory. I would have hoped and expected that the "get_details()" method would allow each activity module to add details of it's own custom completion criteria. However, as we have seen, currently only a couple of common criteria are recognized.
To get this report to behave as you expect, it would be necessary to modify the "completion_criteria_activity.php" file that I linked to earlier. As it is part of core module, it would take a lot of determination to get it changed. Furthermore, each activity module would then need to be altered to give details of its completion criteria. Another possibility is to commission your own report and add it as a plugin to your Moodle site. Neither path is short or easy.