undo leaves a character

This is a dangerous bug. It came with the last versions of pspad I think.
I have windows 7 64 / pspad 54.5.9 2503

It happens with large and empty files.
Unfortunately not reproduceable - it occurs only sometimes.

Right now I started PSPad, opened a php file with this content:
<?php
echo htmlentities("&");
?>

Then I did like this:
navigate to end of 2nd line, press enter,enter,enter,enter,
type "skgljsdlfjglsjdflgjsdlfgk"
press "enter,enter,enter,enter,enter"
type "afgnskdngknsdgn"
press "cursor up,cursor up,cursor up,cursor up,cursor up"
and then hold down CTRL-z

The file loses the mark "edited" but the letter "s" remains.

Edited 1 time(s). Last edit at 2015-03-09 09:55 by phpdev93392.

Build 2503 is pretty old - from the 09/2014
Please download the current version and recheck your bug

Now It happened again. I have version 4.5.9 (2600).

Do you use real tabs?

tab key and tabbed files, yes. but that should not be an issue.