switchport mode trunk not showing in running configuration

[ring_gt]

Hi,
We have s 6506-E that needed to be powered down due to a maintenance.
After it booted up, we noticed that the configuration "switchport mode trunk" disappeared from the 
configuration under one of the port channels (including its member interfaces) , even though "show startup" shows it is there. We confirmed the configuration in the show startup is the one that was loaded after the reboot. 

show startup result 
interface Port-channel4
switchport
switchport trunk allowed vlan 197,198
switchport mode trunk
no ip address

show running result
interface Port-channel4
switchport
switchport trunk allowed vlan 197,198
no ip address

Aside from this, the operation of the etherchannel is working fine, and is displaying as a trunk.
It is not a big issue, I am just wondering if there is a bug or something.
We are running on an old IOS version as well:

IOS (tm) s72033_rp Software (s72033_rp-IPSERVICESK9_WAN-M), Version 12.2(18)SXF9, RELEASE SOFTWARE (fc1)

[Abheesh Kumar]

Hi,
I think it may be a bug, If you have maintenance window try to upgrade and see but check this Field Notice: FN - 63743 before doing upgrade.

HTH
Abheesh

[Reza Sharifi]

Hi,

It appears to be a cosmetic bug as the EtherChannel is working fine. 

The IOS you are running (12.2(18)SXF9) is over 10 years old.  So, is it probably time to upgrade.

HTH