I had a similar issue on macOS 27.2 and found a fix in my case. It may be worth checking if you have ever renamed your user account or home folder.
The affected folders opened correctly, but their icons in the Finder sidebar would not return to normal. Removing the remaining Synology Drive components and rebuilding the user’s IconServices caches did not fix the sidebar icons.
We eventually checked this file:
~/Library/Application Support/com.apple.sharedfilelist/com.apple.LSSharedFileList.FavoriteItems.sfl4
Inside the file, the bookmarks for Pictures, Movies, Music and Desktop still contained my old home folder path. For example:
Saved path: /Users/oldname/Pictures
Current path: /Users/newname/Pictures
Interestingly, macOS could still resolve these bookmarks to the correct current folders, and it did not mark them as stale. The folders opened normally, so the old paths were not obvious.
After backing up the file, we used the macOS Foundation APIs to generate fresh bookmark data for those folders using their current paths. We replaced only the affected bookmark data, keeping the sidebar item order, UUIDs and other properties unchanged. We then restarted the current user’s sharedfilelistd process and Finder.
The sidebar icons returned to normal and remained correct after another Finder relaunch.
Important: this is a binary property list containing an NSKeyedArchiver archive, not a plain-text settings file. Please do not simply search and replace the username in a text editor. Make a backup before making any changes.
I cannot say whether this is the cause in your case, but if your account or home folder has been renamed, checking the saved bookmark paths may help.
Kaimo & Guanzi