Looking forward to new insights! :)

You forgot to highlight Kaliningrad exclave on the map

Yes, of course, it's just that these spring fires start from burning grass in most cases, unlike later in the season, when dry thunderstorms have a huge impact. And there are also many cases of fires originating from logging cites, but Transbaikalia is just not really one of them.

Mostly open larch and birch forests, alternating with steppe or wetlands (in higher / more northern areas)

There's very little commercial logging in Transbaikalia since the forests have low tree density and are dominated by larch, less valuable compared to other Siberian tree species. The main cause of fires is locals setting fire to dry grass in these open woodlands out of negligence or stupidity :(