Still image default duration not working when I change Frame Rate

I am making a frame by frame animation which needs to be 12fps. In preferences, I set the still image default duration as 2 frames. Then I set up the sequence as the Digital SLR 1080p 24fps setting. I then drop in my frames which auto sets to 2 frames per image.

The timebase for this preset is actually 23.976. When I go to sequence settings and change the timebase to 24fps, the frames that I drop in are no longer auto set to 2 frames as the still image default duration, they are put into the timeline as 1 frame. I can't seem to find a way around this - the default in preferences is still set to 2 frames but it always auto sets to 1 frame.

Why does the still image default duration not work when I change from 23.976 to 24fps?