Sorry it's taken a while to get to this. The behavior Christopher reported is a bug, which is fixed by the patch attached to this message.
Christopher, please note that the intended behavior is that you are prompted to dissuade you from editing the bullet character. If you elect to change it manually, that change will not actually affect the bullet, and the next time the item is decorated (like, after it's hidden and then exposed) then the character you manually edited will be changed back.
Generally, you could instead deactivate allout-widgets-mode and edit the topic bullet that way. Unfortunately, there's another bug which misses "undecorating" the first topic in the buffer, if it's at the start of the buffer. I've submitted a separate patch to rectify that:
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=42218.