My employer mandated a minimum number of days in-office per week and then implemented the flex desk system with all of the drawbacks that you outlined. In my company's case it's a little worse: There are, in fact, often no available open desks in my workgroup. Furthermore there's no enforcement or discipline with respect to day-of desk reservations.
So this means that about half a dozen employees have permanently camped out at a "flex" desk, essentially claiming them for their own. Everyone I guess "knows" that those aren't real flex desks in practice, or at least nobody wants to stand out as the asshole who kicks Richard off his de-facto personal desk.
For the desks that aren't permanently camped nobody bothers to use the online tool to actually reserve them. They just show up at some point during the day, and if there's no jacket on the chair or backpack under the desk or something, they just sit down at it and start using it. If you reserve the desk in the tool before coming into the office often you'll arrive to find someone sitting there who hasn't bothered to use the tool to reserve it. So then you don't have a desk at all and need to camp out in a common area on your laptop, surrounded by 3 or 4 other people blabbing away on Zoom calls.
If you confront them to kick them off the desk they'll usually move, but that's a really uncomfortable thing to do and makes you look a little like a bully to everyone around. "Dude, you snooze you lose. Dick got there first. Why don't you just go to the common area where all the other late-coming slackers have to go?"
It's a productivity hellscape that I do everything in my power to avoid. I've resorted to dropping by the office a few times a week just to badge in and grab a bite to eat from the microkitchen so that I show up in the query that management runs once a quarter to make sure everyone's badging in often enough. I also make it a point to walk prominently by several other desks and saying "hi" to people to give the illusion that I'm "in the office" on a semi-regular basis.
The first time management tries to turn the screws to deal with my behavior I'm triggering my plan to move on to a role in another company that's fully remote.