I've used the swap set previously, but when I set my Frontal layer as default it just kept Frontal visible while continuing to do whatever funky nonsense it was doing by keeping my Quarter position as the default then switching as I turned my head.
Are your heads tagged correctly on the right side? I can see from your first screenshot that your Frontal group is tagged as "Frontal" on the right, but what about the others? Also, do you have any manual triggers that could be conflicting here?
If you expand the Head Turner behavior in the Rigging workspace you can see all the groups assigned to each head turn pose; see if that gives you any clues.
And lastly, you don't have to have the Head Turner behavior on the Head layer itself, so if none of the above help, remove that behavior and move it to the top-level puppet, just to see what happens.
Sorry for my delayed response.
Because of time constraints, I went ahead and completed the rigging for the head. Meaning I have layers for all five positions now. I mirrored the 'finished' heads to the opposite side.
Whatever I did during that time seemed to fix the problem. The head position defaults to Frontal whenever I reset the character in Record mode.
I'm thinking the program automatically chooses the middle most layer in a group of head positions. That's my guess for what happened, I hadn't bothered to test that theory though.
To answer your questions.
Each of the original three heads were tagged appropriately.
There weren't any triggers. I added triggers at some point to fix my issue before posting here. The triggers were causing a completely different problem, so I removed them entirely.
I don't believe moving the Head Turner behavior changed anything. I recall moving it from the top to the bottom and back again, with no visible change to the animation.