Collection Type as "Blank" Doesn't Stay Set When Applied to Topics in Ditamaps

After getting some unexpected results from using a Sequence collection type in a ditamap, I attempted to clear the Collection type field to blank, but could not change the setting back and have it "stick." (I can change the setting and save it, but it returns back to the orginal Sequence setting.)

The unexpected results were a mixed list of subtopics that started with the first half bulleted and the second half numbered. This might be because the topics are a mix of concept and task topics?

There have also been some problems with XMetal locking up as well, so could this be more a system problem than software problem?

Any insight appreciated,
Diana