If you choose Ordered List from the SubNote Type menu, the list maxes out at 10 items. If you add more than 10 items, ArchivesSpace deletes items. Sometimes AS deletes just the middle section of items (image 1) and sometimes it deletes random items and reorders the list (image 2).
10 items seems to be the breaking point, but I've been able to get a list of 20 items to work before. However, any additional items still resulted in one of the two outcomes.
In this image I had entered 15 items in numerical order. This is what happened after I saved.
In this image I had entered 25 items in numerical order. This is what happened after I saved.
Edit: In the first image I had entered 20 items in numerical order.