It's Other Explorer MRUs located in:
Cleaner > Windows (tab) > Windows Explorer > Other Explorer MRUs
Untick Other Explorer MRUs to retain the MRU history.
This is what it contains which is removing the RegEdit MRU:
[Other Explorer MRUs]
ID=1011
LangSecRef=3002
LangRef=3124
Default=True
RegKey1=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32
RegKey2=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FindComputerMRU
RegKey3=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\PrnPortsMRU
RegKey4=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Map Network Drive MRU
RegKey5=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\ComputerDescriptions
RegKey6=HKLM\Software\Microsoft\Direct3D\MostRecentApplication|Name
RegKey7=HKCU\Software\Microsoft\Direct3D\MostRecentApplication|Name
RegKey8=HKLM\Software\Microsoft\DirectDraw\MostRecentApplication
RegKey9=HKCU\Software\Microsoft\DirectInput\MostRecentApplication|Id
RegKey10=HKCU\Software\Microsoft\DirectInput\MostRecentApplication|Name
RegKey11=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\WordWheelQuery
RegKey12=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\TypedPaths
RegKey13=HKCU\Software\Microsoft\Windows\CurrentVersion\Applets\Regedit|LastKey
RegKey14=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Wallpaper\MRU
RegKey15=HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Wallpapers\Images
Hmm, upon looking into this for you I realized that's why my wallpaper folder is always forgotten. Would be nice if Piriform separated some of those MRUs for a more tailored configuration.