While working on all kinds of Flash projects I've seen many different ways of masking the irises – some using a duplicate version of the eye whites symbol without the outlines or two copies of the same frames on two separate layers. If a symbol is used as a mask, Flash only reads the shapes that are placed on the bottom layer of that symbol. The other layers are ignored. I've never seen this documented as a feature. But it doesn't seem to be a bug either.
All methods that require the animator to deal with 2-3 separate instances – the eye whites and the mask (and sometimes the outline) make adding new frames, i.e. eye expressions a somehow complicated and non-intuitive process.
I always prefer to work with lines instead of fills outlines whenever possible. It makes modifications and shape tweening a lot faster and easier to do. Of course, if thick-and-thin outlines are required this does not work.
In the example above the same symbol was used for the eye whites and the iris mask. The outline is twice as heavy as the other outlines, because only the portion that appears outside of the mask is actually visible in the composite image.
The fill of that same symbol is used as the iris mask and a white color card is used as underlay.
It may sound complicated, but is actually one of the simplest and most elegant masking tricks I've come up with. Download the sample file to study how it was made.
Edit: 11 March 2020
Another method of masking is demonstrated in the video below: