I am using the user registration plugin and created my own registration. (all working fine)
I created a subform field with a single existing text field that can multiply. (works fine)
I created a few fields in the rsform text_1, text_2 and a hidden field to grab the two fields together. I used javascript and combined the two fields formatted as json into the hidden field which I then mapped to the user custom field I created. the javascript runs on submit (all works)
So everything works but the last step. the content of the hidden field will not write to the subform field, even when formatted as how it is stored in the database when it is written directly in the backend user edit.
when I look at what is submitted in the RS FORMs table for that field, there is content. Nothing appears in the database 'XX_fields_values' for that field.

now I am wondering is there support for subform fields in RSForms? Because everything is working as expected but it wont write to the database.

I have now confirmed that it's not necessarily the subform type field. I created a test text field, mapped it, and that doesn't work either. no new fields are writing to the database.