That's because management points in secondary sites are not full replacements for MPs in primary sites; clients must still be able to communicate with an MP in the primary site. Secondary sites are *not* gateways so what you are seeing is by design.
You should replace the MP in the sub domain with a site system that hosts the MP, DP, and SUP roles (no secondary site). Clients will then prefer (or fail-over) to these roles.
Also, AD doesn't really provide any info about which MP to use. It provides an MP for the clients to use when the client doesn't know about any MPs and this in turn is only used to determine which MP to continue to use -- a bootstrap process.
Jason | http://blog.configmgrftw.com | @jasonsandys