To be honest, the lightbox (which is a third party library) doesn't have great support for RTL, but I think there could be something else at play here related to CSS caching.
For example if I switch to RTL it breaks, but then if I do something which rebuilds the CSS cache, then it seems to work better... not entirely convinced it's doing the correct things in terms of the directions things move etc. but it certainly it's "less broken" in that things don't disappear off the screen.
Sometimes I can switch back to LTR and now LTR is broken.
So, I think the lightbox itself is working correctly, but perhaps something is off with our CSS cache.