runtime.ContextType.MAPREDUCE is incorrect. runtime.ContextType.MAP_REDUCE is the correct one. API doc says it is MAPREDUCE which doesn't work. Just found out when the UE was not triggering for MR context.

the api docs say

runtime.ContextType.MAP_REDUCE

set the string to

'MAPREDUCE'

After your comments I realized I was looking in the wrong column. Thank you @Sandii