Rewrite rule ignored by webpagetest?

[Mushu]

In our Windows Server 2019 IIS 10 site's web.config using HTTP/2 protocol we have a rewrite rule:

Code:

<rule name="Prioritize Brotli" enabled="true">
    <match url=".*" />
    <conditions>
        <add input="{HTTP_ACCEPT_ENCODING}" pattern="\bbr(?!;q=0)\b" />
    </conditions>
    <serverVariables>
        <set name="HTTP_ACCEPT_ENCODING" value="br" />
    </serverVariables>
</rule>

And yet the Request for many objects still shows this, for example:

Code:

origin: https://my.domain.com
:path: /cms/content/fonts/Montserrat/Montserrat-ExtraBold.otf
accept-language: en-US,en;q=0.9
accept-encoding: gzip, deflate, br
sec-fetch-site: same-origin
accept: */*
user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.141 Safari/537.36 PTST/210106.200150
:scheme: https
referer: https://my.domain.com/content/css/dark.min.css
sec-fetch-mode: cors
:method: GET
:authority: my.domain.com
sec-fetch-dest: font

What gives? I understood the rewrite rule will change the incoming accepted encoding to just show "br", isn't that right?