Just chiming in here for those waiting for an answer. They just resolved this on the arbitrum side because it was happening for arbitrum transactions as well. Turns out Arbiscan's API does not report the correct gas price, meaning this actually isn't a straightforward problem to fix on Koinly's part. I'm near positive it's the same issue for Optimism. I would recommend also petitioning the team at optimistic.etherscan.io to fix their API

Any update on this? Optimism imports are unusable as-is.

Yeah it is calculating them wrong. It looks like it is just calculating L2 gas price x L2 gas used, when it should be (L2 gas price x L2 gas used ) + (L1 gas price x L1 gas used x L1 scalar). Thus the fees are all grossly underestimated

Took me ages to realise why my ETH didn't square. This is the reason.

It's more than just the decimal though.

The following is the same transaction. The top transaction was CSV import, the bottom was synced from address.