Ae becomes Æ

I have an action group that translates defined by me substitues of illegal characters into illegal characters. Also I have defined them in my "Tag - Filename" icon. I use them to copy the name of the song from FILENAME to TAG title and from TITLE to FILENAME

All of my my "translations" work as they should. With one exception:

TITLE >>> FILENAME
Æ >>> Ae
æ >>> ae

FILENAME >>> TITLE
Ae >>> Æ
ae >>> Æ

So it seems, there is a bug that makes lover "ae" into upper "Æ" [instead of lower "ae"], when coping from FILENAME to TITLE

[un-checking the "case sensitive comparison" did not help]

Please show us the action script code for the related conversion rule.

DD.20150423.1806.CEST

In the .MTA file it looks the same

But in the Mp3tag itself it looks different

Did you try the replace option "case-sensitive comparison" ?
Does it give the expected result?

DD.20150425.1713.CEST

Now it seems to be working fine

And the .MTA file show a distinction:

1=Ae
2=\u00c6

1=ae
2=\u00e6

I just don't know what happened between that day and today