Dear Eriv,
Rather than Tracking Code, please using Last Touch Channel Details(Hopefully you have captured tracking code there while setting up Marketing Channels).
If you still see that the tracking codes attributed to different channels, then there is some problem with the Marketing Channel Setup.
But as far as i know, if you breakdown Marketing Channel by tracking codes, it wont show the exact results and as predicted by you earlier, it is due to expiration only.
Even though you change the Tracking Code expiration same as Last Touch Channel, it wont help.
Assume a case where the visitor came to your site for the first time on 01 Jan 018(Based on first touch, the expiration is 30 Jan 2018).
If the same customer visit your site again on 10 Jan through email with Tracking Code EM(Tracking Code Expiration is now 09 Feb 2018) and again on 20 Jan through Natural Search.
If you go to last touch and break down Natural Search, Tracking Code EM will be there though the last touch is Natural Search.
Since both the expiration works independently, it wont work.
Best way break down the Marketing Channel with Marketing Channel Detail.
Thank You!
Arun