Special characters are replaced by Replacement Character in all Rest and GraphQL communication #5909

GET and GraphQL calls (received by and send from our backend) suddenly contain replacement characters for special chars (German ä,ü,ö, the paragraph char,...).
This behaviour started last last week Monday in multiple microservices. Logs show that just before json/request answers are returned, the text is as it should be.
What is weird is that this behaviour starts showing after about 30 minutes give or take. When the kubernetes pod is restarted it works again (no replacement chars) until it doesn't.
We compared the package versions in the faulty release of our service with the last working release and the only package that changed was axios (see list below). We tried running a service with the old (and working) axios version but the error still happens.

We are using the same express version (4.19.2) for a long time and this just started happening. I assume it is one of its dependencies but have no clue how to figure out which one it is.
I have installed Express versions 4.18.2 and 4.17.2 in our services and they run as expected (no errors). The following is a list of all packages that our microservice uses:

+-- @types/axios@0.14.0
+-- @types/cookie-session@2.0.49
+-- @types/cors@2.8.17
+-- @types/express@4.17.21
+-- @types/jest@29.5.12
+-- @types/js-yaml@4.0.9
+-- @types/jsonwebtoken@8.5.9
+-- @types/pg-format@1.0.5
+-- @types/pg@8.11.8
+-- @types/uuid@8.3.4
+-- axios@1.7.7
+-- cookie-session@1.4.0
+-- cors@2.8.5
+-- express-async-errors@3.1.1
+-- express@4.19.2
+-- js-yaml@4.1.0
+-- jsonwebtoken@8.5.1
+-- moment-timezone@0.5.45
+-- moment@2.30.1
+-- mongodb@4.17.2
+-- pg-format@1.0.4
+-- pg@8.12.0
+-- pm2@5.4.2
+-- ts-node-dev@2.0.0	
+-- uuid@8.3.2
+-- typescript@4.9.5

Any help/hint where to look is much appreciated.

Thanks