Cannot select "List Default" in /mailman3/users/$USERID/manage

1. I went to a site I maintain
2. returned to the top page
3. clicked on a list
4. clicked manage on a user (in both cases, just the one closest to the mouse ponter, no systematic idea)
5. clicked on the moderation option (which was List Default), selected Accept, clicked Update => option remained Accept
6. clicked on the moderation option (still Accept), selected List Default, clicked Update => option reverted to Accept
7. clicked on the moderation option (still Accept), selected Hold, clicked Update => option remained Hold.

I tried a few different things, but the pattern was consistent: I could change the option to any value except List Default.

I'm guessing that the problem is that "List Default" is not an option in the menu in the page template.