According to the documentation, a Groovy map created using Map#withDefault should add unknown keys referenced via Map#get or Map#[] to the map itself. When executing the below code snippet as a Jenkins pipeline, unknown keys will not be added to the map.

def map = [:].withDefault { [] }
map.get('A') << '1'
map['B'] << '1'

println map.toString() // prints [:], expected [A:[1], B:[1]]
println map.keySet().toString() // prints [], expected [A, B]

Running the same snippet in a regular Groovy interpreter or the Jenkins script console will result in the expected output.