I have a trouble ticket open on this subject now, though I've asked support to hold on it until I could run a few experiments.
This issue was resolved in 18.104.22.168, but apparently returned somewhere along the line in the 4.1 series. After close inspection however, I'm not absolutely certain that it's the exact issue that was present in 4.0.x < 13.1. The lightbox function seems to have something to do with screen resolution. On desktop computers with resolution 1920x1080 and lower, only images posted since the upgrade show in a lightbox. Any images which displayed correctly in a lightbox in 3.4.8 open in a new tab.
On mobile devices with higher resolution screens -- I've tried Ipad air 2, Samsung G Tab S 8.4, Nexus 6, Surface Pro 3 -- the lightbox seems to work correctly on images in landscape orientation, but when an image is in portrait mode, and it's height is > 720 pixels, the whole mechanism seems to break down. After attempting to view a portrait image that fails, all "legacy" images fail; even those that opened in a lightbox just moments before. It's a weird one for sure, and I haven't finished collecting data on it yet.
I'll post back when I'm able to tell IPS support exactly what's going on.