If you prefer getting HTTP 200 instead of HTTP 204, use an image instead of nothing in the HTTP API. See there for the send_image parameter.
For the tracking issue, maybe your browser sends a DNT HTTP header parameter, then Matomo does not track it. Try with another browser and/or configure this in Matomo:
https://fr.matomo.org/docs/privacy-how-to/#step-4-respect-donottrack-preference