Hi Niels,
It is definitely a bug and I have noticed what is causing it.
I rarely use the keyboard shortcuts as the laptop remains on my desk while the tablet is on a standing platform allowing me to stand while I animate. I do however have a small number pad next to the tablet (I would love to be able to map this in the future but I digress)
Anyway, I have noticed the cause!
I use the sidebar to toggle the lightbox. And when I am too lazy to reach down to the laptop to flip the pages, I will instead flip pages with the timeline. It is this particular action that causes the lightbox to cease working. It will no longer toggle on and off after flipping drawings with the timeline and instead remains off, prompting a relaunch of the app (after saving the scene).
As for the other glitch I witnessed, that was just a once off. I think it was taking a minute to load the scene and it detected just one frame until it was fully loaded, so you can disregard that one.
I can send you a video of the bug I have found if you wish?
Hope this is helpful