Alright, I answer it myself! 
As inspired by the DHCP problem as mentioned in this post http://forum.mikrotik.com/t/ros-6-38-serious-dhcp-server-problem/105259/1
I suspected the same problem happened in this scenario.
BINGO! same STP problem also applied here and I resolve it by adding the admin-mac attribute to the “bridge1”, all the things solved at once.
It seems that the STP problem introduced in 6.38 interfere the RouterOS in many aspects. 