JRACLOUD-69783 deals specifically with creating and updating issues. The substance of that ticket is “it seems like accountId works everywhere else, but not for creating issues.” You should use “accountId” here.
I appreciate that this is a bit confusing (including, as I just noticed, that the REST API documentation does not specify the attributes for UserDetails for sending notifications…), and so we are working to allow “accountId” to work for creating and updating issues as well.