Since Flow 6.0 the Browser $arguments were accidentially moved to the request query parameters, while they should reflect the post body arguments. This is fixed with this release (see #2050), but note that this slightly changes behaviour if you adapted your usage of Browser since 6.0.0. If you want to send query parameters, you should instead provide an Uri instance with the parameters, as was already the case prior to 6.0.
See changelog for details.