Tactical Mode Testing
After a couple of days of trial-and-error testing, I have come to the conclusion that <ApplyToTile> does not work in Tactical mode, hence why the spell Wall of Fire is commented out in the fire-tactical spell xml. Some have noted that there is a <GameModifier> in the CoreTile TD_Groundfire which might help it work, but this GM does nothing (as far as I can tell) on either the Tactical or Strategic layers.
Strategic Mode Testing
<ApplyToTile> does work to some extent on the Strategic layer: it only triggers the GameModifier when a unit moves onto the applied tile. If the unit does not move from the tile, the GM does not trigger at the beginning of the next turn, so, for example, a unit could be sitting in the middle of a raging inferno, yet not taking damage. Of course one could use a <PerTurn> DoT type effect to help alleviate this for damage type effects. I have yet to discover a work-around to this movement issue. <ApplyToTile> also works in all affected tiles when the spell is set with a Radius > 0.
I will keep testing and post more conclusions if I come across any.