chaining blockContent() filters do not work as expected

[szuecs]

blockContent("A") -> blockContent("B")

does not work as expected and resulted in

[APP]time="2023-09-18T14:31:29Z" level=error msg="error while proxying after 1.20231ms, route kube_.. with backend lb http://10.2.54.204:8080, status code 500: dialing failed false: unexpected error from Go stdlib net/http package during roundtrip: net/http: HTTP/1.x transport connection broken: http: ContentLength=807 with Body length 8999, remote host: ..., request: \"POST /api/foo HTTP/1.1\", host: foo.domain, user agent: \"\"" trace_id=71ab8e710720a5b6

[szuecs]

this is now fixed and tested by #2828