drop down Fields remain editable by all participants

Hi all

Strange issue. I have a template document setup with the creator to prefill all of the data, and then 4 participants to simply sign off the file.

There are around 100 fields in the form, and majority are free text input. The sender prefills all these and the data in all these free text fields is locked from that point onwards. 

There are approx 20x drop down fields that are also for the sender to prefill. These drop down fields are remaining 'unlocked' and editable by anyone in the workflow. 

i have checked the fields and they are all assigned to 'sender-prefill' only.

Anyone come across this and any ideas how to fix?