Passing some parameters in OIDC redirect URI

Question

I'm using OIDC login with AuthorizationCode flow, and everything works fine, except I have to pass some parameters in the redirect URI.

OIDC authentication tokens (code, state) are passed in POST.

In the Querystring of the redirect URI, I pass some url-encoded parameters, e.g.:

https://test.auth0.com/authorize?response_type=code&
    nonce=...&
    state=...&
    code_challenge=...&
    code_challenge_method=S256&
    client_id=...&
    scope=openid%20profile%20email%20api&
    response_mode=form_post&
    redirect_uri=http%3a%2f%2flocalhost%3a60000%2fDefault.aspx%3fAppToOpen%3dA01

Correctly calls me back to:

http://localhost:60000/Default.aspx?AppToOpen=A01

Passing one parameter is fine, but when I send more than one:

https://test.auth0.com/authorize?response_type=code&
    nonce=...&
    state=...&
    code_challenge=...&
    code_challenge_method=S256&
    client_id=...&
    scope=openid%20profile%20email%20api&
    response_mode=form_post&
    redirect_uri=http%3a%2f%2flocalhost%3a60000%2fDefault.aspx%3fAppToOpen%3dA01%26BodCode%3DSO_2

It redirects to this URL:

http://localhost:60000/Default.aspx?AppToOpen=A01&BodCode=SO_2

As you can see, %26 is correctly decoded to &, but then is encoded to &.

How to avoid that re-encoding?

Or, is it possible to have some parameters passed in POST instead?