I was at Bethesda Marriott and got my 500 points for MAGC for 1 night (July 2018), everything worked as it should. Sheraton Uptown ABQ did also post the points too.
However I didn't have a good experience at a hotel in Germany, it's new and here's the ordeal:
First stay: saw the MAGC in their tab (instead of the hotel information booklet) and called the front desk and they claimed they have it and as long as I put out my DND they will credit me the points. I did and no point arrived and wrote to the hotel and they said they are not participating.
Second stay: They had the hangtag so I used it, I took a picture of it. At check-out I reminded them that I had my hangtag out and he asked me for the card that stuck under the door which there wasn't. Of course no point was awarded and had to chase it with MR.
Will have a stay there again later this month and let's see how it goes.
Here's a list of MAGC hotels in the US and Canada:
List of MAGC hotels in the US and Canada