Scripting: Replace in "Comment" (.m4a)

Tags: #<Tag:0x00007f7570299ae0> #<Tag:0x00007f75702999f0>

I want to replace somenting in the (normal) comment field of .m4a files:
$set(comment,$replace(%comment%, Op. , op. ))

This doesn’t work. Nothing will be changed (as I see it in Picard).
The log view looks good, but nothing is changed
D: 17:06:59,349 file.update:614: Updating file <MP4File "1-01 - Auf's Korn! - Kopie.m4a">
D: 17:06:59,352 file.update:614: Updating file <MP4File '1-02 - Frühlingsstimmen - Kopie.m4a'>
Also after save thre is no change in the files.

Just to set something to the common field works:
$set(comment,Lorem ipsum)
I see the changes in Picard.

What is wrong?