Display Always On not working as expected

My MK3i watch face is really nice and I want on all the time not just when I look at it - otherwise nobody but me sees it.

I realize this will impact battery life, but am willing.

I set

Settings -> Appearance -> Watch Face -> Display Settings -> Always On Display = ON

But it won't stay on. After the 15 second timeout it reverts to AOD or blank screen.

How do I control this?