I have gamerule doFireTick set to false in my world. On upgrading to 25w02a it is still set to false, but fire is now spreading. (I also have randomTickSpeed set to 0, fwiw.)
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More detail: I had a log on fire in a room, and several wood items a few blocks away. Those blocks caught on fire by the time I noticed them.
When you say that fire is spreading, do you mean stuff like fireballs and items still being able to create fire?