The key field should be an array instead of a comma separated string.
https://github.com/linkedin/feathr/blob/ff66b2a3cd4966f8e79aa0e7f92b705f1eede46d/feathr_project/feathr/query_feature_list.py#L24

Corresponding Scala code reading this value is at:
https://github.com/linkedin/feathr/blob/ff66b2a3cd4966f8e79aa0e7f92b705f1eede46d/src/main/scala/com/linkedin/feathr/offline/config/FeatureJoinConfig.scala#L56