400 bad request on set participant data with nested custom vars

stuck on setting participant data via api. getting 400 bad request with invalid json path.

{
 "participantId": "{{contact.interactionId}}",
 "data": {
 "customVars": {
 "ivr_priority": "{{flow.input.priority}}",
 "routing_key": "{{flow.input.routingKey}}"
 }
 }
}

endpoint POST /api/v2/interactions/participants/{participantId}/data. expression syntax looks solid to me; however, hardcoding values yields the same 400 response. architect expression engine throws error on nested object mapping. why is the schema rejecting customVars structure? downstream data actions require ivr_priority on participant data. timeout set to 5s but response returns instantly. json path validation rejecting this structure for some reason? tried flattening the object to customVars.ivr_priority but that breaks the json schema definition. expecting standard key-value pair acceptance. is there a restriction on nesting depth for participant data actions? also noticed the error message doesn’t point to a specific key, just the root path. feels like a bug in the validation logic for custom variable injection.