Cannot POST ComboOrder after validation success

Hi,

Additional attachments can be found in this support thread:
https://support.imis.com/hc/en-us/requests/192867

I'm trying to make a minimal POST body stub I can use to work with modifying events. To that end I've POSTed (stub.json in the attached support ticket) to /api/ComboOrder/_validate.
I received no validation errors or warnings, and received the validateResults.json in the attached ticket.

I then tried POSTing the "Entity" property returned in the "validationResults.json" to /api/ComboOrder, however I then receive the following error:
https://imis.zendesk.com/attachments/token/UN9FaPNx75kVHEMRyV2nqXfyg/?name=POSTresponse-400validationerror.json

Why is this happening? Shouldn't the object returned from the /validate call work just fine since it's validated?

Thank you