How to set host-header depending on choosen backend when using HAProxy as loadbalancer - haproxy

Question

Under certain circumstances it is required to modify the host-header based on the backend selected by HAProxy loadbalancing (an example is described here: https://www.claudiokuenzler.com/blog/919/haproxy-how-use-different-http-host-header-for-each-backend-server)
My internet research shows that there are basically two ways to achieve this:
a) use "http-send-name-header Host" (not recommended according the HAProxy docs: https://www.haproxy.com/documentation/hapee/latest/onepage/#4.2-http-send-name-header)
b) use " http-request set-header Host ... if { srv_id 1 }"
(the following article describes these two techniques: https://serverfault.com/questions/876871/configure-haproxy-to-include-host-headers-for-different-backends)
Option a) works as expected but it is discouraged to be use (see https://www.haproxy.com/documentation/hapee/latest/onepage/#4.2-http-send-name-header).
For this reason I am trying to option b). Unfortunately the config cowardly refuses to work:
backend loadbalanced-backends
mode http
balance roundrobin
option forwardfor
http-request set-header Host one.domain.com if { srv_id 1 }
http-request set-header Host two.domain.com if { srv_id 2 }
server one one.domain.com:8000
server two two.domain.com:8000
The warnings printed in the log:
[WARNING] (1) : config : parsing ....cfg:14] : anonymous acl will never match because it uses keyword 'srv_id' which is incompatible with 'backend http-request header rule'
[WARNING] (1) : config : parsing ....cfg:15] : anonymous acl will never match because it uses keyword 'srv_id' which is incompatible with 'backend http-request header rule'
Besids the warning the logged request-header show that the host is not changed to one.domain.com resp. two.domain.com.
What exactly means the warning "anonymous acl will never match..."? I do not see what is wrong with the configuration. Any ideas are welcome.