STREAM replacement effective on some HTTP requests but not others

We have a need to rewrite one of the incoming request header fields, and I'm doing that with STREAM profile - it works, but curiously only on roughly half of the requests. The other requests contain the same original string that matches what we want to replace, but get through without being replaced.

The best we can figure is that HTTP requests NOT containing "Content-Length: NNNN" are working for the string replacement, and HTTP requests containing "Content-Length: NNNN" are NOT working for the string replacement.

We tried removing the "Content-Length: NNNN" part from the HTTP request, but that did not work either. "Request Chunking" is set to "Rechunk" on the http profile. Any thoughts on what our problem is, or recommendations on what else to try? Thanks.