I started to notice this with the upgrade to Sonoma. My usual config has the Desktop folder on my Finder sidebar. Every so often, I will go to save a file there (Zoom, Word, PDF, etc.) and Desktop is no longer on the Sidebar as a location.
I add it back in (Finder Settings > Sidebar) and it stays for an indeterminate period of time, and then at some point it’s not there again.
In fact, that’s the only way I’ve added it (manually dragging), but unfortunately it never “sticks” for long. FWIW, I saw this behavior prior to Sonoma. Just added it again now…we’ll see when it disappears again.
I have the same problem, but it’s with Network in the sidebar and it’s on Big Sur. Just today, it came back with no action by me. (I rarely use the Network entry in the sidebar, but I notice it comes and goes.)
It seems as if this is the same problem I had until I disconnected from iCloud Drive. The Desktop folder kept showing up there and disappearing from side bar. Once I disconnected from “Drive” the folder stayed in the sidebar.
Same issue here. No matter how it is added, after a while it disappears from the sidebar. Also Finder windows that are sorted by kind and date descending keep adding new items of the same kind to a new group so the window has to be sorted again to restore the correct view. And a dozen more Finder bugs since Sonoma…