I’m going to assume from some of your words that you are on a Mac, but I don’t know what macOS version or what hardware…
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I don’t have a sure-fire solution to your problem. It’s possible there is a bug either in SketchUp or macOS. But here are some observations about what I see on my Mac.
Have you tried the old-time cure-all of rebooting the system? Sometimes a cached list can get corrupted and other than arcane commands in the Terminal window a reboot is the only way to heal it.
I’m running SketchUp 2018 under macOS 10.13.3 High Sierra on a mid-2012 MBP Retina. I tend to use the File->Recent Items menu in SketchUp rather than the list in the right-click Dock. But when I try the Dock list, I see a different (typically longer) list. It is as if macOS is keeping lists for SketchUp and the Dock separately (or perhaps like SketchUp is keeping its own list independent of macOS). The list in SketchUp seems “smarter” in the sense that the one in the dock includes files I have since moved or trashed. SketchUp’s seems to have purged such dead files from its list. I haven’t found a way to force them back into sync.
I also notice that the list can “overflow”, that is, when it fills to the max set in Preferences->General older ones are forgotten. If I then move or delete some files, the list does not refill with the older ones (not surprising: forgotten is truly forgotten).