[CapedCrusader]

Tell them it's because while it's on the purchase list, it's doesn't belong to them yet and therefore doesn't apply Rank yet LOL.
No, really though, that's kind of what it is. Although I'm surprised it's coming up blank on the purchase list and not showing the original d4.

The main reason is because a Staff is a hand weapon, and those build their damage using the value in the Damage Die field. The wpDamage field mainly used for Ranged Weapons. The trick here is that the Weapon Die value also determines the minimum Strength. That being said, your trick of leaving the Weapon Die value blank is what makes this work for you. That's one of the things it reads to know when to build it or use the wpDamage value. So that's what's making it work, without the WeaponDie it's using it like a ranged weapon and using the wpDamage value.
So, three ways to go:
1) Leave it like it is, and have your players live with it.
2) Set the timing on this earlier. I'm not sure right off-hand if you can get the Rank value before this is set in the price list. That may limit what you can do.
3) Modify Weapon Die rather than wpDamage. You'd need to take care of the Min Req Str though (field[wpStrReq].value).