Cisco – MAB and 802.1x Issue – MAB-authenticated device gets dropped

I'm trying to use 802.1x to authenticate clients on my network with dynamic VLAN assignment from RADIUS. We have IP-Phones(powered by PoE) that only supports EAP-MD5, and we would rather use MAB(it also uses LLDP-MED for some settings) to authenticate the phones using the MAC-range from the phones vendor. The following scenario works perfect:

• Connect the phone and let it boot up(takes a while) and authenticate with MAB.
• Connect a computer in the phones data-port and let it authenticate with 802.1x(or fail and reach guest-vlan)

However, the following scenario doesn't work:

• The computer is already connected to the phone
• The phone is then connected to the switch

What happends now is that the computer is authenticated using 802.1x before the phone boots up and get's authenticated with MAB. When the phone is ready, it's authenticated with MAB and everything works. However, after a short period(let's say a minute), using debug authentication all, we see a "NEW LL MAC: phones mac" message(which is weird since the mac has already been MAB-authenticated), and then we are unable to contact the phone using ping. When I check show mac address-table it has now moved the mac from Port Gi 0/12to Port Drop. However, if I check show mab interface Gi 0/12 or show authentication sessions it lists the phones-mac as mab auth sucess.

Can anyone explain why the first scenario works, and not the second?

The switch is a 3560E PoE 24p with IOS 12.2.58SE2Sample of the switch-config.:

network-policy profile 1
 voice vlan 90
!
interface GigabitEthernet0/12
 switchport mode access
 network-policy 1
 authentication control-direction in
 authentication event fail retry 1 action authorize vlan 60
 authentication event server dead action authorize vlan 60
 authentication event no-response action authorize vlan 60
 authentication event server alive action reinitialize 
 authentication host-mode multi-domain
 authentication order mab dot1x
 authentication priority mab dot1x
 authentication port-control auto
 authentication periodic
 authentication violation replace
 mab
 dot1x pae authenticator
 dot1x timeout tx-period 5
 dot1x max-reauth-req 1
 spanning-tree portfast
!

Btw, when we tried authenticating the phones using 802.1x too (EAP-MD5), there are NO problems in any of the scenarios. However, we want to use MAB instead of 802.1x to avoid the requirement of configuring the phones with a username and password.