Why don't I see my custom group?

In one of my arcs, I have an ambush group of bosses taken from several standard groups. The rest of the enemies/groups in the arc are properly balanced, and xp is not a problem. This ambush group has its own name and is an official enemy group.

In addition, some of the bosses from this all-boss ambush group are also bosses in the main mission group. The custom bosses in the main mission group are specific characters that are set to not auto spawn, and I don't have room to add another custom boss specific to this group, so I used bosses from the ambush group to take up the slack.

The problem I'm having is that this group is not showing up on certain drop-down lists. For example, the group does not appear in the main selection for boss detail; I can't pick a boss from this group at all for a boss instance (though I can make one of them appear as a boss detail by picking one of their dopplegangers from the main mission group). However, the group does show up in the drop-down list for "surrounding enemy group".

These are the only two instances I can remember off the top of my head; across the rest of the drop-downs, they appear in some, but not in others. I can say that I have another all-boss ambush group I'm using in another arc I'm working on, and it appears and disappears on all the same lists.

I had this problem before, long ago (at least, as far as having a group not showing up in some menus, I can't remember if it was all bosses) and I fixed it. But it was so long ago, I have no recollection what I did to get the group showing up again. My arcs can get pretty cluttered, and I've had issues where I thought I deleted something but there was a small reference elsewhere that brought the whole kit'n kaboodle back. But I've saved my published arcs to local files, examined them, and have found no such incongruities (yet).

Anybody have any ideas why my group doesn't show up? Is it possible that the AE is actively blocking it because it's "all-boss", and with "surrounding group" set to "same as boss" it would be an all-boss spawn? ...Even then, I'm still able to select this group for "surrounding group", since it shows up in that list... So I'm just hornswoggled on this one.

You can't set a boss objective with a custom group if it only contains standard enemies. You'll have to take the bosses out of the standard group they are actually from for the boss objective and rename/reedit them for the specific boss fight.