From a technical perspective, what happens when canceling an already placed buy/sell order?

As mentioned here, PSBT represents off-chain transaction expectations; therefore, the removal process is simply a delisting, making the PSBT no longer public. If a third party has already obtained the PSBT, they can still fulfill the PSBT and complete the transaction even after it has been delisted. In other words, removing the PSBT does not render it invalid.

To truly invalidate a PSBT, the method is simple. The user just needs to initiate a transaction to themselves, sending the assets signed in the PSBT back to their own wallet.

In future updates, the Orders will consider adding an "invalidate PSBT simultaneously" option in the delisting user interface. Users would only need to sign a transaction to themselves with their wallet to complete the process.