User App 4.7.2 upgraded from 4.5

Used to be, if you want to do a SOAP call to Start Workflow (do-start-workflow token) you need to provide in the soap call, every single form field by name, but not neccasarily with a value. I.e. There are 30 fields, but this call only needs to fill in 12.
In 4.5 that worked. In 4.7.2 the same PRD has 30, but is sending 15, but in fact the do-start-workflow call has all 30 defined, sent as "" basically.
Since the do-start-workflow token does NOT show in trace the actual SOAP message (That would be a neat enancement) it is hard to know what is actually sent.. But watching in Catalina trace, with sufficient logging enabled, we see that only 15 data items are sent. And we get an error that a required field is missing.
Is this a change in the behavior in 4.7.2? And if so, how do you send an empty field in the SOAP call now?