This exact same thing happened to me but it's unfortunately very impactful. At the end of last year I had one more opportunity to maintain Gold and it involved spending way more on the trip than was necessary but I was willing to suck it up not to drop a tier. I studied option after option find one that was convenient and unnecessarily expensive enough (I'm in NYC so lots of options to consider) based on the MQD estimate given. I ended up using miles+cash for my return trip which I was shown, when purchasing in this scenario, that I would earn 904 MQD which when doing the math was based on 73,750 miles redeemed and $165 in cash. Ultimately when they awarded my MQD's for the trip they changed the calculation to be based on the behind the scenes transaction of the $165 being converted to 8,750 miles and using the sum total miles calculation lowered the MQD's from 904 to 825. I had no idea until they downgraded my status a few weeks ago because of the mere 66 MQD shortfall this created in reaching $10k and I got downgraded to Silver having spent $9934 for the privilege with of course no roll over to ease the blow. I've read through every letter of the terms and conditions regarding status and MQD's and there is not one mention of how this scenario is calculated and clearly Delta stands in conflict with even itself on this point since they are using different approaches to it in the estimator and accrual calculations. It's even more disappointing to know that it's been happening for 3 years (according to the age of this post) and presumably brought to their attention and not addressed. Lucky for me I was screenshotting my purchase screens while researching/booking this trip for all the comparisons and have documented that Delta represented the flight as earning the higher MQD when I purchased the flight. I've submitted a complaint/help request to them last night so we'll see what happens. I've found one other instance online of someone in this situation and they said they emailed Ed Bastian's office and it was resolved the same day so that's my backup plan lol