Hi.
Most of my stays at Marriot Bonvoy hotels are through travel agent. In most of the cases I get my nights and points credited. AC Milano is the hotel where I always get it. In May the stay (2 nights) was not credited and I asked for credit. I submitted copy if invoice (for extras). After few days I got reply that they will check it. Few days ago I got answer that my stay is not eligible for nights because is was rate for group. Ok, no problems with that. I just noted that in my last few stays I had similar reservation and nights were always credited.
Just few minutes later my 2 nights from February stay were deleted and I got message from Alessandro from Marriott Bonvoy Case Resolution Specialist that "nights in February have been adjusted". So so far I had 27 nights in 2022, I asked for 2 more in May (promotional period) so I should have 31. At the end I had 2 less than before - 25.
For sure my worst guest relations experience ever.

Not sure if you can call this a poor guest relations experience. You got something which you were not even entitled to get, since booking through a travel agent.

Its definitely poor customer service. If you accidentally give a customer something, you should never take it away. I agree with not crediting the new stay because it shouldn't qualify.

I had an issue in January where I tried to pay for a Points+Cash stay with gift card, and the +Cash portion never went to the hotel. Problem was, once they applied the gift card to my account, they couldnt figure out how to take it off. Without me knowing, they added a 4th night to my reservation, charged $50 to my gift card (the balance of it) for the 4th night (much better than my $300 +Cash portion). I was happy with this resolution since I saved $250.

Did I stay this 4th night, no. Would definitely be annoyed if they took it away now though. In addition, I was target for the promotion of stay 1 cash night and get 1 35k FNA. I didn't technically qualify for the night or the award, but once issued they better not take it away.

As it stands though, I am more than happy to have saved $250, get an extra night credited and get a FNA.

AFAIK this depends on the details of how the travel agent booking is done. OLTA bookings don't earn, but for example refundable FHR reservations made through AmEx PTS/CTS do earn. I suspect that the OP uses a real live human travel agent, so correct application of the rule could go either way.

Yes it depends on the rate. For example, TAs can book the Marriott Stars rates, which absolutely should credit. Same with Hyatt Prive rates. OTOH, most discounted rates will not qualify.

if a cashier gave you the wrong amount of chnage I return for a cash purchase, say a significant amount, then they would surely be right to ask for it back. So I disagree with your statement in general terms. Just need to explain why it was taken back.