From 1.12.3
Quote:
A formation undertaking an engage action is allowed to make one move (not a double distance move as is the case in many sets of wargame rules, not least many Games Workshop games), and then fights an assault against the enemy formation that was chosen as the target of the charge. This move is known as the charge move.
Make the move normally, as described in the movement rules given previously. Once the move is complete, the engaging formation must have at least one unit within 15cms of a unit from the target formation. If this is not the case then the assault does not take place and the action ends.
As I said earlier, IIRC this whole aspect was discussed many years ago by NealHunt, and formed the basis for his complete review of section 1.12. The point being that certain aspects were implied from earlier / later sentences in the section.
Elsewhere in 1.12.5 units must be within 15cm AND have LoS to be able to fight. There are situations where opposing units could end up within 15cm but be outside LoS. This causes no casualties by direct combat, and the rules move on to allow each side to call on support. This effectively triggers the exact case you refer to, which I agree that we should avoid.