[wkharris2001]

the judge,

what it actually sounds like is the op is a US citizen with a UK spousal visa that is expiring. a us citizen is allowed a maximum stay of 6 months in the UK without a visa, after that a visa is required. it sounds to me like the agent at check in plugged in the information of the passenger being a u.s. citizen, traveling to the UK and got the following results on Timatic

Passport required.
- Passport and/or passport replacing documents must be valid
for the period of intended stay.

Visa required, except for A max. stay of 6 months:
- for nationals of U.S.A.;

at check in the agent is required to verify visa requirements and passport validity. the OP was denied boarding based on the fact that they are not a permanent resident of the UK and their visa expires prior to their return to the U.S. so legally at this point they are not allowed to stay more than 6 months. now for delta to cover their butt with immigrations in the UK they have to make sure that all passengers they are sending to the UK have all of their documents in order. so the only way to do that, was for the ticket to be reissued to say that the passenger was not staying longer than 6 months. this also probably helped the passenger when they got to the UK to get through customs without a lot of interrogation.