I just realized that, while training, if I switch to another tab to do something else, the training stops. When I switch back, it continues normally. The only thing that continues if the tab loses focus is the time counter.
I think the expected behavior would be to continue the training even if the tab doesn't have the focus.
I'm using:
Windows 10
Firefox 64.0.2 (64-bit)
Dozens of tabs opened.