Strange Apache CustomLog behavior

Question

I'm adding a Docker healthcheck function which I've successfully done in one container, but adding the same code to a different container is behaving strangely.

Essentially, I'm simply trying to not log certain requests, using an env var.

In dockerfile:

HEALTHCHECK CMD curl -ILfSs http://localhost:80 -A "HealthCheck" || exit 1

RUN ln -sf /dev/stdout /var/www/logs/access.log \
    && ln -sf /dev/stdout /var/www/logs/transfer.log \
    && ln -sf /dev/stdout /var/www/logs/ssl-transfer.log \
    && ln -sf /dev/stderr /var/www/logs/error.log \
    && ln -sf /dev/stderr /var/www/logs/ssl-error.log

In httpd.conf:

<IfModule log_config_module>
    LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
    LogFormat "%h %l %u %t \"%r\" %>s %b" common

    <IfModule logio_module>
      LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %I %O" combinedio
    </IfModule>

    <IfModule setenvif_module>
       BrowserMatchNoCase ^healthcheck nolog
    </IfModule>

    CustomLog "logs/access.log" combinedio env=!nolog

</IfModule>

So now when the healthcheck ticks, the log which would normally show this:

127.0.0.1 - - [05/Oct/2022:03:22:13 +0000] "HEAD / HTTP/1.1" 200 - "-" "HealthCheck" 74 1095

shows this (it shouldn't be showing anything at all since the env var gets set when the UserAgent matches 'healthcheck'):

127.0.0.1 - - [05/Oct/2022:05:17:46 +0000] "HEAD / HTTP/1.1" 200 -

It acts almost as if it's doing a replace of everything that matches 'healthcheck' and after, with nothing at all (empty string), which I don't think it's even supposed to be capable of doing(?).

I've tested to make sure that the only healthcheck being called is this one in httpd.conf by commenting out the BrowserMatchNoCase ^healthcheck nolog line, and it indeed then just shows the usual, expected line containing the "HealthCheck" useragent.