Back Story
I created a private calendar back at the beginning of January called "Ward Youth Council". I assigned all members of the bishopric and the presidencies of the Young Women organization, Young Women class, and Aaronic priesthood quorums by calling. I saved the calendar with no problem, but I didn't check to see if the YW class presidency callings were still there at that time.
Fast forward to today when I went in to double check that all of the correct members were assigned before I created an event. Imagine my surprise when I found all of the other callings listed with the members assigned in LCR except the YW class presidency. My first thought was "How could I, as the Bishop, forget to include the YW class presidency on this calendar?" So, quickly repenting of my error, I added the entire YW class presidency, except the class secretary (I know, right?

I saved the calendar without any errors appearing and began to create the event. Just before saving the event I decided to triple check myself and went back to the Manage Calendars page to ensure that all of the correct callings were listed as (at least viewers) of the calendar. I read the list, re-read the list, and then re-re-read the list and no YW class presidency callings appeared.

Believing I was losing my mind, I re-added the Young Women class presidency callings and saved the calendar. When I re-opened the Calendar Details page again, poof, they were gone.

What began next was a frantic test of the calling assignments.
- I removed all of the AP quorum callings, saved the calendar, and then re-added them. No problems there.
- I created a new private calendar and added just the "Young Women Class President" calling to the list of editors and tried to save the calendar. No luck. The calendar never saved; I only saw the "Give us a moment..." message at the bottom of the page, but the calendar never saved.
- I then added another calling to this new calendar and was able to save, but when I went to view the list of assigned callings, you guessed it, the YW Class President calling was gone.
So, long story short, it appears that the following callings cannot be added to any calendar:
- Young Women Class President
- Young Women Class First Counselor
- Young Women Class Second Counselor
- Young Women Class Secretary
I added all of the YW class presidency by name, saved the changes, and verified that they do in fact appear on the list now; though still not by calling.