When you click on a screenshot, it opens in a pop-up. If there are multiple screenshots, there's no way to navigate between them from the pop-up. You need to close it and open it again for the next one.
This was fixed yesterday in master.
Excellent! Unfortunately master segfaults for me: https://bugs.kde.org/show_bug.cgi?id=387829
I'll confirm after that's resolved.
There are buttons now, and that's good. Still no keyboard navigation. I'll track that with https://bugs.kde.org/show_bug.cgi?id=387858
previous/next arrows are available even when only one screenshot is available.
Can you file another bug for that?