I have noticed that if I set the --compressed option in Curl the file does not get truncated. I searched on Gerrit and found a Varnish configuration change from April 11 whose commit message says:
Note we still need to re-test assumptions about do_stream=false adding Content-Length to responses which lacked them in the common case (no gzip/gunzip on the fly on the response side).
Commit here: https://gerrit.wikimedia.org/r/#/c/282716. Related tasks: T128813, T131501, T131761