Is it? They're each deciding, independently, to handle their electors in a certain fashion. They each have the right to independently change that decision, without any penalty for doing so. It's not a binding agreement with another state in any fashion.

> OTOH, pacts like the popular vote compact signed by some states in the past few years is almost certainly unconstitutional.

Why would that be the case? Electors are not bound by any particular (federal) rules in how they cast their votes. If we look at the popular vote compact as a friendly handshake agreement (to which electors are not legally bound), they are free to fulfill the handshake agreement and vote along with the national popular vote.

Of course, if a signatory decides last-minute to betray that agreement, there perhaps would be no legal remedy.