In the graphql_map_input_fields_to_wp_query filter, the $post_type is the last parameter. This is inconsistent with the other graphql_map_input_fields_* filters. For example in graphql_map_input_fields_to_get_terms the $taxonomy parameter is the third parameter (while the $context & $info parameters are the last two in all of the graphql_map_input_fields_* filters). For consistency sake, it may make sense to update this filter.

wp_query filter:

get_terms filter: