So one might think the issue is that lighbox always creates a block element, but that’s not so simple:
Consider this markdown:
Some text![50x50](https://images.unsplash.com/photo-1526047932273-341f2a7631f9)Some text![50x50](https://images.unsplash.com/photo-1526047932273-341f2a7631f9)Some text
To be a bug, per the category description, this must prevent normal use of Discourse. I don’t see this preventing anyone from using Discourse in a normal manner. Recategorizing to feature.