Description of the Issue

After using Notepad++ for a while (maybe a day or two, but probably depends how much you actually do within it), a single Undo operation (using Ctrl-z) will undo a large number of actions. So you have to "Redo" to at least not lose a lot of work, but it's impossible to then just undo the last one or few changes. Restarting Notepad++ fixes it temporarily, until it breaks again. I see this at least twice every week. It's not new - been happening for at least the last 1-2 years on multiple computers (Win 7 and Win 10, all 64-bit).

Steps to Reproduce the Issue

1. Use Notepad++ for a while. Probably requires at least a day or two, but may just be a certain number of particular actions.
2. Press Ctrl-z.

Expected Behavior

The last change is undone, but nothing else.

Actual Behavior

A large number of changes are undone.

Debug Information

Notepad++ v7.9 (32-bit)
Build time : Sep 22 2020 - 03:24:22
Path : D:\Programs\Notepad++\notepad++.exe
Admin mode : ON
Local Conf mode : ON
OS Name : Windows 7 Home Premium (64-bit)
OS Build : 7601.0
Current ANSI codepage : 1252
Plugins : ComparePlugin.dll mimeTools.dll NppConverter.dll NppExport.dll NPPJSONViewer.dll Runme.dll XMLTools.dll