According to the API docs:

"The value of the mergeable attribute can be true, false, or null. If the value is null, this means that the mergeability hasn't been computed yet, and a background job was started to compute it. Give the job a few moments to complete, and then submit the request again. When the job is complete, the response will include a non-null value for the mergeable attribute."

This casting currently treats "null" as "can not merge", which cascades down to other plugins. Should the returned type be expanded?