I have a similar issue that seems to be caused by OOTB BR called "Read only Model when not initial state".
The problem I have with this BR is that I created a RP for a change request which needs different processing that other changes. Meaning different than Standard, Normal and Emergency. I have created a new model with 4 states and set transitions between them.
The chg_model is set in script part of the record producer. When the RP is submitted the change model is set correctly but user is displayed Change model cannot be modified for in progress Change Requests error on the portal.
The BR is supposed to run "on update" but it triggers on new records. Interesting part is that values user provided in the RP are listed as Update number 1, not Update number 0.
