`opfSubstitutions` work only partially

[BlueCutOfficial]

I tried to work around issue #61 by using opfSubstitutions to add some properties="svg" attributes in the <manifest> of my content.opf file.

opfSubstitutions:
    - regex: <item id="ch002_xhtml" href="text/ch002.xhtml" media-type="application/xhtml+xml" />
      replacement: <item id="ch002_xhtml" href="text/ch002.xhtml" media-type="application/xhtml+xml" properties="svg" />

It doesn't work.

I tested the example in the README:

opfSubstitutions:
    - regex: <dc:language>fr-FR</dc:language>
      replacement: <dc:language>fr-BE</dc:language>

This one does work, so it seems the replacement works for part of the content but not for my manifest items.