[DiapDealer]

As far as I can see, the file type isn't really relevant. Nor is the sequence. Just the simple act of adding a file to an epub, AND writing to an existing xhtml file in the same plugin seems to be enough to make things go pear-shaped. Doing one or the other alone alone, doesn't.

Your test3 plugin that you used to verify hunspell/gumbo stuff was working in plugins causes the issue for me as well (at least on Windows). Using the Bundled Python3 OR External Python3

I'm pretty sure there is a launcher-code bug here somewhere, but I haven't been able track down where.

EDIT: Whatever it ends up being, it seems that writing to an existing file with a plugin seems to bypass the Preserve Entities logic.