Count me in.
Although mine seems to be a little different than eveyrone elses. I don't loose all passwords on all devices at the same time. Each device seems to be on it's own forgetting schedule, where it forgets after 3 to 7 days.
Also I do use iCloud Keychain which is supposed to sync preferred wifi networks and passwords. But on Macs, you can see the list of preferred networks and they are clearly not in sync at all. It's like something glitched and each mac/device keeps reverting back to it's own saved state without wifi passwords. You add them, they're good for a while, then they go away.
I've also been working with apple support on this issue but I got absolutely nowhere. Chat support sent me to phone support, phone support sent me to senior phone support, senior phone support won't send me to an engineer and keeps suggesting troubleshooting ideas that are not even remotely on the right track. I'm tempted to abandon my support ticket and start a new one from scratch.
I've experienced this problem on a 2012 mac mini, 2009 imac, iPhone 11, iPhone 7 plus, iphone 6s. The ONE computer that has never had an issue seems to be my primary Mac, a 2018 mini that stays on wifi and it's "Preferred Networks" list in system prefs is a mile long and contains everything.
I've tried turning iCloud Keychain off and on multiple times.
I've tried FULLY logging some of the machines out of iCloud, deleting all data, and then logging them back in.
It makes no difference at all. I'm at my house and my mothers house the most, and both networks constantly disappear from computers/phones.
Apple senior phone tech tried to suggest the problem was the wifi router at one point. Ironic since both routers on those two networks are genuine airport express base stations :D
This is *CLEARLY* an iCloud Keychain problem. Which means only Apple can fix it.