I was refused a price below my own partner price. Why?

Because it was your first paying restaurant. Pricing below your partner price is allowed once you already have at least one OTHER restaurant on a paid plan — the shortfall gets netted off against your future payouts, and with no other paying restaurant there is nothing to net it against. Both halves are doing work in that sentence: "other" means not the restaurant you are pricing, and "on a paid plan" means genuinely paying, because a free trial produces no payout to absorb anything. Each pricing card carries a badge telling you which side of the line it is on. Once the floor lifts there is no cap on how far you may discount.