I've had what you experienced happen to me with EC, but today another odd email.

Last week the paid first class option was cheap, so I bought for my trip, and my fare class changed to G. Today I get an email that I've been upgraded to first class on the return (even though I'm already in first class) and when looking at my reservation noticed my fare class on the return is now RU.

It's like Delta wants me know that if I hadn't bought the upgrade I could've been sitting in first for free

My receipt still shows G class in both, so we will see if I get the bonus MQM's or not.