Thanks Wolf.
I noticed that my xml file had an extra glob pattern line.
I'll reboot my desktop to see if there is any difference.
Here is my freedesktop.org.xml.

Before:
<sub-class-of type="application/x-raw-disk-image"/>
<alias type="application/x-iso9660-image"/>
<glob pattern="*.iso"/>
<glob pattern="*.iso9660"/>
</mime-type>


After:
<sub-class-of type="application/x-raw-disk-image"/>
<alias type="application/x-iso9660-image"/>
<glob weight="80" pattern="*.iso"/>
<glob pattern="*.iso"/>
<glob pattern="*.iso9660"/>
</mime-type>

My problem is fixed.
Thanks so much everyone!
I'll report this to Debian.

kde-pcdoctor wrote:After:
<sub-class-of type="application/x-raw-disk-image"/>
<alias type="application/x-iso9660-image"/>
<glob weight="80" pattern="*.iso"/>
<glob pattern="*.iso"/>
<glob pattern="*.iso9660"/>
</mime-type>

You can remove the red-colored line.
It is actually being replaced by the ' <glob weight="80" pattern="*.iso"/>' one.

I did notice that line.
Thanks again!

kde-pcdoctor wrote:I'll report this to Debian.

FYI, here's the corresponding KDE bug report, and a statement of the dolphin maintainer:
https://bugs.kde.org/show_bug.cgi?id=337035#c44

The guy in the freedesktop.org bug report did claim this is a dolphin/KDE bug (and fixed it anyway), but it doesn't seem to be.

Thanks again Wolf. I try other desktop environments like Cinnamon or XFCE but keep coming back to KDE.