The idea to use aggregate is correct. But in this case, to get the count of issues with specified labels you should aggregate issues instead of labels and for each issue validate assigned labels. For aggregation, you may use function Count() and iterate through issues using function Descendants()
NonZero(
Count(
Filter(
--iterate through issues
Descendants([Issue].CurrentHierarchyMember,[Issue].[Issue]),
--which have label FWQA_TestDev
([Measures].[Issues created],
[Label].[FWQA_TestDev]) > 0
--and don't have lable FWQA_Triage
AND IsEmpty(
([Measures].[Issues created],
[Label].[FWQA_Triage]) )
)
))
Best,
Zane / support@eazyBI.com