Fortinet transparent mode (devices can not reach each other)

Dear All, 

             i'm new in Fortinet, and I have a question would like to seek for your assist, current i am deploying a fortinet 101F model to our customer, and that fortinet 101F are configured as transparent mode behind a internet firewall, a HPE server connected to fortinet port 1, and port X1 interface which is running 10G via a Fiber cable connect to a H3C switches as uplink, actually the foritnet is just working as a LAN switch, and you might question why it needs to use Fortinet, you can use just use a L2 switch, the reason of this is customer has a Fortinet and want to make use of this device, and the issue now is when i connected the X1 internet 10G fiber to the H3C, its affected the network some devices wont be able to reach each other, and when i tried to disable the uplink X1 interface, the issue disappeared right away, just wonder what setting caused such issue ? 

  And i have one throught, can i simply create a firewall policy, one is in-interface (port 1) and out-interface (X1) so that i can enable security profile in this firewall policy in it, any help would be appreicated 

Keith