Although fences have a narrow hit box lava does not flow into the block the fence is placed at, resulting in a gap where mobs can survive. As expected the wooden fence will burn after some time and the lava now flows again.
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And how would you suggest fluids to work? The only possible way is through block-based occupancy, for a number of reasons.
The textures need to be done so there is no seam
Hitboxes, etc. can vary slightly, due to technical limitations. There may be a minute space between ajacent block's hitboxes for this same reason, floating point error.
Many designs that have already been created depend on these quirks of liquid mechanics.
But if you think you can do a better job with the liquid mechanics, go ahead and make a mod. Smooth lighting originated as a mod, and it is part of the game now. Mojang could use someone like you if you can get a realistic fluid system set up.
Also, a mod will probably reiterate this, but do not mark future versions as affected. "You don't have access to this yet."
Basic minecraft mechanics.
Every non-cubic block leaves a gap to the next block.