G729ab (annexb=yes) support

I have two SIP endpoints registered with FreePBX and both support G729ab. I also have configured FreePBX so that there is direct RTP streams between the endpoints (canreinvite=yes). I found that FreePBX will modify the INVITE when forwarding to the remote device to remove G729b (annexb=no) even though the endpoints specified ‘annexb=yes’ in the INVITE sent to FreePBX. Since I have direct RTP media between the endpoints why does FreePBX feel the need to remove G729b support in the INVITE? Is there a workaround for this?