I have created a dynamic module that has a related data field with images. I also have a form that users fill out to create the data item in the module and also upload a photo that will be related to the item. When my form submits and I checked the data item in the backend and my related image was not showing. After checking the database and my content items I saw that my date item was being created under my custom provider (which I use for a custom url) but my content link under the "OpenAccessProvider". This difference caused the related media to no show on my content items.
TL:DR - When relating data make sure that your providers for the data item, the related item, and the content link are all the same.