This sounds like an obvious question but I can't find the answer anywhere (sorry if I've missed it):
A squad in an otherwise OG hex is claiming WA. An enemy squad, in its APh, enters that hex, and the CC results in a Melee. If the Melee is subsequently fired on across the wall, it obviously affects both units, but who gets WA? Only the unit that was originally claiming WA, or both of them? Does it matter whether the enemy squad entered across the wall in its APh or not? B9.321 just talks about adjacent units of opposing sides, not same-hex.
The opposing units in the same location must forfeit WA per B9.322 because they no longer fulfil the requirement for maintaining WA of B9.32 - in this case not being in the same Location with a non-hidden, non-prisoner enemy unit.
Thus the non-CC/Melee units will be the only ones benefitting from WA.
B9.32 WALL ADVANTAGE (WA): A unit may claim WA over a same level wall/hedge hexside if it is an armed, unbroken ground level unit which is
not: a vehicle eligible to receive in-hex TEM of ≥ 1
[EXC: Height Advantage (10.31)/Cactus Patch (14.7)/Olive Grove (14.8)], in Column/Convoy, in a Fortified building possessing a Gun, on a bridge
[EXC: over a Roadblock], in a pillbox/cave, beneath an entrenchment counter, above Wire/Panji or
in a Location containing a non-hidden, non-prisoner enemy [EXC: "broken" vehicle (A12.1)] unit
B9.322 A unit claiming WA must always be marked with a "Wall Advan" counter. Placing a "Wall Advan" counter and claiming WA are synonymous, as are removing a "Wall Advan" counter and forfeiting WA [EXC: 9.323]. A Pinned, TI, or Immobile unit cannot voluntarily claim or forfeit WA. Claiming WA is voluntary [EXC: 9.323], and can be done by a unit at five times: during its setup; at the end of any RPh (step 1.32B of ASOP, ATTACKER first); during its MPh/APh (either as part of, or before/after MF/MP expenditures); when losing HIP status; whenever all enemy units lose/forfeit WA over shared wall/hedge hexsides. WA must be forfeited immediately if a unit no longer fulfills 9.32 and may be forfeited at any other time.
von Marwitz