STR:
Open a new document (UTF-8).
Paste some text.
Convert to ANSI.
Select All.
Delete.
Undo.

Result:
Doc is marked as Saved.

Notepad++ v8.3.3 (64-bit)
Build time : Mar 13 2022 - 17:20:02
Path : C:---\npp.8.3.3.portable.x64\notepad++.exe
Command Line :
Admin mode : ON
Local Conf mode : ON
Cloud Config : OFF
OS Name : Windows 10 Pro (64-bit)
OS Version : 2009
OS Build : 19042.985
Current ANSI codepage : 1255
Plugins : BetterMultiSelection.dll DSpellCheck.dll mimeTools.dll NppConverter.dll NppExport.dll