Folders with custom icons are now plain folders post-upgrade
I have a collection of hundreds of folder icons which I've copied onto drives and devices for years. I am transferring my data to a new MacBook Pro with Monterey. While installed software and their files display with their icons, the previously-customized folders are now generic (all the icons are gone). The icon folders are collected in my Documents folder and were downloaded from iCloud from the old Mac. (They all still show as custom icons on the old Mac.)
What happened to those folders, and can I still use old icon images with the new system?
MacBook Pro 14″, macOS 12.0