Windows IIS ARR Reverse Proxy Encoding Issue

Question

We have an environment with an Windows 2019 Server IIS 10, which is acting as Reverse Proxy (ARR) for my IIS Server farm (Application Request Routing 3.0 and URL Rewrite 2.1). We send the users name in the HTTP headers. But my ARR somehow screws up the encoding (we are using german special characters, e.g. ö,ü,ß...), so when i check the respone of the WebServer it shows me: H%C3%B6lmuth M%C3%A4%C3%9Fterm%C3%BCller instead of Hölmuth Mäßtermüller.

I have an old environment with Windows 2012R2 Server with the same configuration, in this environment the display of the name is correct. I have checked all kind of settings between old and new servers, but cannot find any difference.

Futher i have used Failed Request Loggins and Network Monitor to see what the server receives and sends, below find the results.

Received Request on the IIS ARR (old and new):
X-AUTHENTICATE-FamilyName: M%C3%A4%C3%9Fterm%C3%Bcller
X-AUTHENTICATE-GivenName: H%C3%83%C2%B6lmuth
X-AUTHENTICATE-cn: H%C3%B6lmuth M%C3%A4%C3%9Fterm%C3%BCller

Request send to the IIS (new ARR):
X-AUTHENTICATE-FamilyName: M%C3%A4%C3%9Fterm%C3%BCller
X-AUTHENTICATE-GivenName: H%C3%83%C2%B6lmuth
X-AUTHENTICATE-cn: H%C3%B6lmuth M%C3%A4%C3%9Fterm%C3%BCller

Request send to the IIS (old ARR):
X-AUTHENTICATE-FamilyName: Mäßtermüller
X-AUTHENTICATE-GivenName: Hölmuth
X-AUTHENTICATE-cn: Hölmuth Mäßtermüller

Below find also the Rewrite we use to Route the Request to specific Server Farms:
<rewrite>
<globalRules useOriginalURLEncoding="true">
<rule name="ARR_BPBP-DEV_loadbalance" patternSyntax="Wildcard" stopProcessing="true">
<match url="" />
<action type="Rewrite" url="http://BPBP-DEV/{R:0}" />
<conditions>
<add input="{HTTP_HOST}" pattern="bmi-bpbp-dev.vecos.at" />
</conditions>
</rule>
<rule name="ARR_BPBP-TEST_loadbalance" patternSyntax="Wildcard" stopProcessing="true">
<match url="" />
<action type="Rewrite" url="http://BPBP-TEST/{R:0}" />
<conditions>
<add input="{HTTP_HOST}" pattern="bmi-bpbp-test.vecos.at" />
</conditions>
<serverVariables>
</serverVariables>
</rule>
</globalRules>

Anyone has an idea how i can change this behaviour? Help would be much appreciated

Regards,
Alex