I've also encountered the "NaN" for numbers issue. It happened to me from spamming the basic attack "swipe" as a dweller. I would assume it was caused by clicking too quickly and not letting whatever it was in the background.
I hoped that introducing a delay to the buttons would fix this issue but it seems like that's not the case. Were you playing on mobile by any chance?
No, I was playing on my PC in my Firefox browser.
