Hey, quick question, is your node built with declarative style (using routing) or programmatic style (with execute method)?
I’ve seen a similar issue before where expressions inside collection fields don’t get evaluated in declarative nodes. The UI shows the correct value but when executing it just passes the raw expression string to the API.
There was another thread about this: http://community.n8n.io/t/expressions-in-collection-fields-not-evaluating-need-workaround-or-best-practice/189191
The guy there ended up rewriting his node in programmatic style and that fixed it. Not ideal but it worked.
Would be interesting to know which style you’re using.