There have been a couple of known causes for this, and once they are known they have been getting fixed (reference my post/reply above, ).
I don't think that at this time we have other known root causes identified.
Unless the sequence of events responsible happen in close proximity to eachother, then it is difficult to know exactly when the cause for this occurred on the system as the inability to add the field to the portlet tends to happen at a later and separate occasion.
So I would say not yet on the list of items to be fixed anymore or for now, but it will return to that list when we can identify another root cause for it.