UltraEdit supports multiple clipboards and I suspect you have activated one of the user clipboards.
The clipboards are activated from the Edit menu, Right Click Menu or Ctrl+0 - Ctrl+9 with Ctrl+0 being the standard clipboard. The active clipboard is shown in the status bar following the line/col separated by a comma. C0 reflects the Windows Clipboard and C1-9 the user clipboards.
Another possible cause of confusion in relation to the clipboards is a setting under Advanced -> Configuration -> Editor -> Miscellaneous. If the option Automatically copy to clipboard when selection is made is selected, this would cause any selected text to automatically overwrite the current contents of the clipboard when selected.