Thanks a lot! I've opened https://github.com/Palats/mapshot/issues/38 to track that.
I'm wondering what's going on here - I would have expected rendering of furnace flame to be mostly based on the state of the game - and given the deterministic nature of Factorio, extra delay would not matter. OTOH, maybe the flame is just some asynchronous cosmetic thing. Also, looking at my old example at https://mapshot.palats.xyz/?x=-51.4&y=-46.6&z=5.1 , there is actually a mix of on & off furnace - I imagine largely based on what the activity of the furnace is at this instant. I could also imagine that even a fully loaded furnace might have a single tick off here and there, in between getting rid of its content and loading more. I'll need to test a bit I think.