I contacted j2store guys and there is very high probability that the issue below is related to the shopin template. In particular Site offline. J2store carousel when in mobile view doesn't allow numkey edit in quantities. As soon as I try to enter a value it throws off the process. Video of problem with voice-over: https://www.youtube.com/watch?v=DjEc3LVsiCI

So then it seems to be related to joomshaper shopin template since I tried it to a fresh install of shopin template (no extensions etc) and still the problem was there.

What may be wrong?