Fleece is an unusual one because in its normal use it is a noun that represents an uncountable substance that must be measured using some other explicit unit. However, this golden fleece example is not its normal use. In this context, there can be "a" golden fleece, and multiple entities which are each a golden fleece can congregate into a collective of golden fleeces which can be counted. So I would side with the quest tracker on this one.

Do you happen to have an exact quote from the NPC so we can better debate whether "fleece" or "fleeces" would be appropriate for their dialog?