When the Object is created without publishing it, it's possible to add the Object in the Form and publish the Form but it is not possible to Submit the Form.
Steps to Reproduce
- Create an Object with a date field
- Go to Content & Data > Forms > Add new Form
- Go to Settings
- Go to Select a Storage Type and put Object
- Select the Object
- Add a text field
- Go to all Advanced Tab
- Map the field.
- Publish the Form
- Put a text
- Submit the Form
It's not possible use the Object in Forms without publish
The Object can be add in the Forms
Reproduced In master: