SCI_AUTOCSETDROPRESTOFWORD(1) and SCI_AUTOCSETMULTI(SC_MULTIAUTOC_EACH) do not mix. See https://sourceforge.net/p/scintilla/bugs/2445/

Projects/Src/CompScintEdit.pas

@@ -220,7 +220,6 @@ begin
   Call(SCI_AUTOCSETIGNORECASE, 1, 0);
   Call(SCI_AUTOCSETOPTIONS, SC_AUTOCOMPLETE_FIXED_SIZE, 0); { Removes the ugly WS_THICKFRAME header at the cost of resizability }
   Call(SCI_AUTOCSETMAXHEIGHT, 12, 0);
-  Call(SCI_AUTOCSETMULTI, SC_MULTIAUTOC_EACH, 0);
 
   Call(SCI_SETMULTIPLESELECTION, 1, 0);
   Call(SCI_SETADDITIONALSELECTIONTYPING, 1, 0);

whatsnew.htm

@@ -44,7 +44,6 @@ For conditions of distribution and use, see <a href="files/is/license.txt">LICEN
   <li>Multiple selection now works over Left, Right, Up, Down, Home and End navigation and selection commands.</li>
   <li>Multiple selection now works over word and line deletion commands, and line end insertion.</li>
   <li>Left, Right, etc. navigation with rectangular selection is now allowed.</li>
-  <li>When autocompleting with multiple selections present, the autocompleted text now goes into each selection.</li>
   <li>The Find and Replace dialogs and the tools from the <i>Tools</i> menu which generate script text now all work better with multiple selections present.</li>
   <li>The Find and Replace dialogs now support using Shift+Enter to temporarily search in the opposite direction.</li>
 </ul>