project1537 FastChen That's relatively expected. I mean, for the extension to work images need known tags like [upl-image-preview url=...], [img], or ![].
Other tags (custom BBCode...) can conflict. The benefit of server-side rendering to avoid CLS/DOM issues comes with the trade off that the template must be well defined and therefore doesn't accept just any tag.
My recommendation (as I see you have managed to) is that tasks like centering, which I assume [center] does, should be done directly via CSS using the fancybox classes. This will also give a more uniform result regardless of whether users make use of [center] or not.
