I can reproduce that If the image doesn’t take up the full width of the post.
Edit: if images on your forum are being lightboxed, a possible solution to this would be to add something like this to your Discourse theme’s CSS:
.lightbox-wrapper {
display: block;
}
Discourse is displaying .lightbox-wrapper as inline-block. This is why the caption text can appear to the right of the image. I’m not sure if there will be any issues with using display: block instead. I’ll test it out and see.