Bookmarks simply do not export to XLIFF
It seems that the XLIFF conversion is not really up to par as bookmarks simply disappear when you export a topic with a bookmark to an XLIFF file. If this issue is confirmed, that would be a rather serious problem.
Reproduction steps
1 - Create a topic with a bookmark (no text associated with bookmark in this case, but the same happens if you bookmark text)

2 - export this topic

Result: bookmarks are not exported along with other text in the XLIFF file

