WAN-side VLAN PPPoE disables interface but works

[Cabledude]

Hello all,
I noticed some peculiar behaviour after setting PPPoE for a WAN-side VLAN6: pfSense kicks out the VLAN6 from the "active" interfaces list and pops in a PPPoE "interface" instead. See screen shots below.

Despite what I thought was erroneous the PPPoE connection actually gets established and works happily. I merely don't understand why this is happening. Let me show you what happens:

Step 0: I am setting two WAN-side VLANs to use my SG-1100 for FttH. The SG-1100 replaces my ISP's router:

Step 1: reset SG-1100 to defaults

Step 2: Define VLAN4

Step 3: Define VLAN6

Step 4: VLANs added

Step 5: Enable interface for VLAN4

Step 6: Enable interface for VLAN6

Step 7/8: Tag VLANs 4 and 6 on internal SG-1100 switch (some of you may not see why, but it's a SG-1100 thing - trust me it needs to be done in order to send the VLAN tag into the interface)

Step 9/10/11/12: enable VLAN4 interface with advanced options

Step 13: enable VLAN6 interface with PPPoE protocol

Step 14: apply changes

Now check the interface Assignments list. The VLAN6 interface that had been added in step 6 and which was clearly visible after step 12 (see above screen shots) has now been moved back to the "available" drop down list below:

Still the PPPoE connection is purring along happily, providing fast fiber up/down speeds of 200/250. Please note that the interface name IF_WAN_KPN_INTERNET" is actually the same as the interface that has been demoted to the "available" list.

Again, there doesn't seem to be an issue, but it struck me as quite odd. Any thoughts on this would be most welcome.