Setting a FormParam overwrites all form params from request

[#1]

So I have an AssignMessage policy which is supposed to set a FormParam called "From".

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<AssignMessage async="false" continueOnError="false" enabled="true" name="AM-Set-From">
   <DisplayName>AM - Set From</DisplayName>
      <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
      <AssignTo createNew="false" transport="http" type="request"/>
      <Set>
         <FormParams>
            <FormParam name="From"></FormParam> 
         </FormParams>
      </Set>
</AssignMessage>

The problem is that the request from the client already has 2 form parameters: "To" and "Body". But after this policy is executed in Edge, the only parameter is "From". So Edge is removing the given parameters and only keeping the one from the AssignMessage policy.