[KevinH]

There is no support for a collection tag in the opf in Sigil. Sigil only supports a single rendition. I have never seen this opf tag ever used.

Sigil machine reads the opf and keeps it parseable at all times to speed interactions. So using almost any tool in Sigil will remove any opf collections tag because it views it as a opf syntax error.

I will look into adding support for it but a valid test case would be a big help.

So Please post a properly formatted test case epub. If private, please pm KevinH (me) here with a link and I will look into adding support for it to the opf parser, the plugins, and whereever else it is needed.