I'm not sure if you ever got a reply to this, but up above I shared my (rather frustrating and lengthy experience). Short answer is - you will get them after a couple of weeks and a few calls/chats with agents.

Longer answer - on the agent side, it appears that it looks like the miles were returned, but since there is no pool to return them to; they just sit there. So it takes 2-3 attempts to explain to the agent what actually happened.

Then, they will try to return them, but can't - since they show as "returned" or whatever the term is. So then they have to follow up with their support internally. After a week or so, you may decide to call or email again, and I was actually told that "they were returned and the case is closed" when they weren't.

So I had to restart everything and finally got someone competent and empathetic.

My advice would be - if you need to do this, be prepared for the miles you need to be returned to take a while, so don't be in a time crunch. And take screenshots / pictures of the mileage balance you have before the cancellation in the (probably unlikely) event they try to claim that the miles were already back in your account.

FYI2- I'm a 1K so it made it very easy to get ahold of someone, but if I didn't have the 1K desk I probably would have been 2-3x more frustrated, even though the Mileageplus desk is the one who runs it... the 1K desk would always expedite me. Hopefully this wasn't too convoluted - feel free to DM me if you need any followup info.