Check_http ignores "-4" flag

CMK version: 2.1.0p15.cee
OS version: RHEL 8.7

I’m checking ~30 URLs by the http_check command (Check HTTP Service) and get an average Service check duration of 20ms – 30ms.
The problem I have is that about 5-8 URLs have a service duration time of 5.02-5.03 seconds. The reason for this additionally 5 seconds is the (not working) ipv6 naming resolution. If the command is done on the cli and the “-4” flag is set the duration for the “problematic” URLs goes down to the expected 20ms.
All the rules are the same way configured - all of them have the “-4” flag set but 5-8 out of 30 seems to ignore this flag.
Thanks in advance for your support!