Just to replicate the issue I have created custom schema for Theme and also one form and navigation hierarchy for this custom schema.
Navigation Hierarchy :
Folder created by using above navigation hierarchy and also after creating folder ,click on advance properties and check for template name for me it's 'kkktheme'. This name have to be same in your schema and form screenshot shared above.
Using form to enter values , result is below. You can see Folder ID is coming automatically and no error for related to folder.
Point to check
1. Make sure you have set default value at schema level and use correct name as the name coming after creating folder using navigation hierarchy
2. In Form use correct model name (case sensitive)
3. In navigation hierarchy use correct schema name
I made these checks again on the schema / navigation item / input form and I confirm you that all the references are correct.
Could you create another folder (folder 2) with the same navigation item and see if the Folder id assigned to your records is different compared to records created within the first folder (folder 1) you created? My issue is that: folder id is assigned correctly when I create the first folder, but when I create another folder with the same model records created here have folder-id of the first folder! Furthermore, in the navigation view I should only see records belonging to the current folder (note: isView = false), not all of them
Please, let me know. I'll try to create them again in the meanwhile.
If you look at folder definition in 'Recipient' or 'Service' schema, Default Folder definition is always specified, i.e if user is not specifying where record will be saved, there is always a default Folder location.
You need to specify default folder location in your 'Theme' Schema and it will work correctly.
How do you assign recipients to specific Folder: By explicitly specifying Folder internal name or primary key. And in case if nothing is specified, then it will be saved to 'Default Folder'.