When accessing a map key via .get() with a var replacement in the key the get operation always returns null.

Example snippet:

def map = ["key_identifier": "test"]

def id = "identifier"
def resA = map.get("key_identifier")
print(resA)

def resB = map.get("key_${id}")
print(resB)
 
This will return in the output:
Test
null