I've been struggling with the same issue ...
HTTP update without modifying the version works fine if i'm on a minor version and making changes to any colomn.
If I start playing with a major version, I end up with a bad gateway and message says "addings to the web site are locked".
When using the HTTP request to change any mandatory metadata on a major version (author, modified ...) The flow will finish correctly but this will ignore the "bNewDocumentUpdate": true parameter. And then the document ends in a next minor version...
I've tried disabling check-in/out and approval required to the library to find out how to unlock the modification. But no luck.
I've published hundreds of documents that have a wrong setting in a column, no way to update that without updating the version to next minor version... and republish to the next major version !
That's a shame.