Draw calls on sprites seem dependent on presence of an alpha channel?

I have a scene that has multiple small sprites sharing the same packing tag.

It generates 1 draw call if the textures are opaque rects.

If I switch one of the sprites to a texture with some transparency, it creates many draw calls.

The new texture has the same packing tag, same size and other settings.

I tried using the 'advanced' texture setting with 'tight mesh' but get the same results.

Anything fundamental I'm missing? I thought the alpha values could be merged if the textures used the same atlas?

Thanks!
Scott

 

To note, I switched out the sprites for quads with cutout shaders and got to 1 draw call without an issue.
I just wish quads had some kind of automatic texture atlas system like the packing tag.