Data loss: media type content reference field displayed as select list

I'm trying to get Media Bulk Upload w/ DropzoneJS working for a custom media type with a Content reference field. When I display the content field as a select list and make a selection on the bulk upload form, the selection isn't saved with the files. However, If I switch that field to display on the bulk upload form as an autocomplete, it saves properly for every file.

Taxonomy term references on the same media type seem to save with the files, whether they are displayed as select or autocomplete.

Is this a bug, or is there some trick for using a content field displayed as a select list that I'm missing?