The compact presumes that the individual states officially publish their popular vote count prior to the EC vote count. An unaffiliated state (or multiple states) can unilaterally declare that they will not officially publish their popular vote count until after the EC meets. The members of the compact cannot provably know the popular vote count; they cannot provably know that they will be casting their votes for the winner of the national popular vote, as required by the compact.
That theoretical possibility is sufficient justification to overturn the whole compact.
Further, if there is every a scenario where the compact actually changes the outcome of an election, any state that would be forced to switch its EC vote will see their populace pushing for immediate withdrawal from the compact, before the EC can even meet. They will have legislation drawn up and ready to sign on election day, releasing their electors from the requirements of the compact.
As a theoretical exercise, the compact is interesting. As a practical alternative to the EC, it utterly fails.
Yeah, the only way the compact survives is if it would not have affected the outcome of an election. If it will have an effect, every red state in the compact will have legislation on the governor's desk by election day, calling for the immediate withdrawal from the compact, before the EC meets. That legislation will be wildly popular among the voters of those states.