I am having exactly the same issue – although setting the plugin (postie) to use HTML instead of Plain Text is not an option for me, as I only want the tiled mosaic gallery and not a repeat of the same images down in the page as well.
I am also using a child theme – just so you know.
Perhaps there’s a custom function that will address this issue.
Specifically, the issue behaves differently depending on whether I post via email or do it by hand.
Part 1:
Galleries created by hand, or when posted via email (with Jetpack’s Tiled Galleries enabled) do not behave like a gallery set – in other words, each image requires a click on it’s own thumbnail in the resulting generated tiled mosaic to view it; next/previous does not apply ‘although it is supposed to.
Part 2: Galleries, when posted by email (with Jetpack’s Tiled Galleries enabled) connect to the Attachment Page ‘rather then to the image file.
Please let me know if you need an admin login and I’ll set you up with one.
Thank you.