The bbcode is set to only accept numeric values. If it doesnt, vbulletin ignores trying to parse it. Observe the POST bbcode tag, which also requires a numeric value:
The fact that it switches to (Purchaseable Content) is because vbcredits tries to prevent visible bare charge tags if for some reason they are (like in a thread preview).
So this is all completely normal and expected behavior, and not a bug, but thanks for reporting it.