Strike prices on common are almost always far far below what investors are paying for preferred shares.

I thought they were priced at the last round price (which is usually a discount if you interpolate the prce between rounds)

Nah, there's a big discount because preferred shares have downside protection (as well as a few other valuable things general) so common stock is valued lower. Generally a 70% discount or so.

the discount varies by investment round. common shares for early stage companies come with a huge discount. or perhaps it is clearer to say that preferred shares come with a huge premium primarily because the liquidation preference (LP) is so valuable at that stage). preferred shares in later rounds have a much lower premium, because investors don't value the LP as much.

I've had this happen, and it's wrong. The price must be adjusted based on the terms. Common stock has basically no terms, so strike price should be much, much lower than what investors are paying for preferred.