The error was related to how I defined the path in the microservice. I initially thought that every request matching the base URL defined in the ingress file would be forwarded to the correct pod, with the rest of the URL being handled by the microservice itself. I simply misunderstood how route definitions in the ingress configuration actually work. The correct path definition for mounting the route is:

app.use("/v1/auth", authRouter);

just like defined in the Ingress config file.

Hope this can be helpful for someone.