A bit weird for me this week.
Twice I asked complimentary upgrades via app chat in advance and was told unavailable.
Both times I arrived to find my rooms have already been upgraded, no question asked.

You can use OTA reward programs to get guaranteed upgrades for rooms; instead of saying you might get upgraded based on "space available", you'll see the upgrade immediately when you book.

Depends on the OTA reward program. AMEX FHR with Four Seasons tends to consistently get me palatial rooms and suites. AMEX FHR with Marriott is hit or miss. I received a great Bonvoy upgrade with FHR during Covid, but the hotel was half empty. My recent experience was a dud.

This matches my experience using Suite Night Awards (SNAs). I'd like to see Marriott add a feature where if you try to use a SNA and it gets rejected, it automatically extends the expiration date by 3 months.