Can't finish a consulted transfer - with incoming call on shared line

Here's the testing conditions:

While on a private line, making a consulted transfer, another call starts ringing in on a shared line.  I am unable to finish the transfer because the incoming call is popped up on my phone and has taken away my transfer softkey. 

If this senerio is tested with 2 private lines, you can hit ignore on the incoming line and regain the transfer button.  Then transfer the call and answer the incoming call.

We did our best to review the config doc and couldn't really find anything to toggle.  We also checked the known limitations of shared lines and didn't seem to find this there either:

https://documents.polycom.com/bundle/vvx-ug-5-7/page/c_vvx_userguide_shared_lines.html

This is on a vvx400 with 5.5.4

Thanks,

Travis