What if someone sends a funding transaction to the project address after the issuance transaction?
Such transaction is ignored by the protocol.
Maybe it's not an issue. If additional funds are sent to the project address after the issuance transaction has occurred that's just more 'shares' that can be issued.
Correct. The funding part is only used for pre-calculating share distribution for the issuance part. Let's say there are two funding transactions: 2 BTC from A and 3 BTC from B. Issuer will than be presented with an option to issue 5 BTC worth of shares (2 to A, 3 to B), but it's not binding or enforced. Issuer can simply decide to issue only 2 BTC worth of shares, say 1 to A and 1 to B, keeping the remaining 3 BTC off the protocol.