[KevinH]

The OPF is and must be machine parseable and regenerable at all times. That is how Sigil's tools work to change the opf each and every time a file is renamed, split, merged, things added, things moved, things deleted etc. Inside Sigil, the opf is actually parsed and stored in data structures to make this possible and reasonably fast. All xml comments are removed from the opf after the first Sigil command that touches the OPF as they are free form and have no place in the data structure. Sigil has always worked liked this. End Users never see the OPF directly nor are they meant to.

You can set an environment variable to turn off Sigil adding its own metadata. And the date modified must always be added according to the epub3 spec each time the epub is modified.

And converting comments to metadata is no solution and is technically incorrect. The OPF as with the nav and ncx are truly meant to be machine parseable. And Comments meant for humans do not belong in files only meant to be machine parseable. Jutoh users or whomever adds that comment can add that metadata if so desired and Sigil will keep it but Sigil will not add it.

And fwiw, that Jutoh "comment" is no longer correct as soon as Sigil open the OPF, cleans and parses it to set up the file. Sigil is the generator of that file from that moment onward.