Not a bug.
Trash a card from your hand. Gain 2 cards each costing less than it.
When you buy this, you may overpay for it. If you do, gain 2 Actions each costing the amount you overpaid.
You may not gain VP cards* when you overpay-buy Stonemanson because they are not action cards. You may gain any card when you trash with stonemanson. The 2nd sentence does not exclude VP Cards in any word. A province does cost less than a platin, everything is fine here.
*Edgecase: you may take a Mill with Stonemanson, because its also an action card.