The sats are still on the mint, they are just moved onchain in the node's wallet. The sats aren't lost per-say but not retreivible over the lightning network. If you trust the mint operator, and he is trustworthy, he will manually redeem the sats for you after you send him the cashu tokens. Remember, you HAVE to trust the mint operator to use his mint, and that means if he deserves your trust he won't rug you.
Discussion
This isn’t quite what I meant. I am referring to the force close fees. I know from experience that you can run a lightning node at a loss. If a mint operator doesn’t charge sufficient fees on lightning transactions, then a force close could cause them to run at a loss (because of on-chain force close fees), which would mean that the mints liabilities over time (ecash) could exceed total liquidity (on-chain + lightning channel) if the mint operator doesn’t provide some liquidity coverage. This is why public auditing may be important.