How Loki Fixes Avengers Endgame’s Biggest Plot Hole

Unless...

While this last part is just for fun, Marvel can absolutely undo any sense of rule-making if they want to fall back on Sylvie's actions from the close of Loki itself.

After Sylvie stabs He Who Remains, we see an infinite number of realities and branched timelines start to occur. Depending on whether Marvel want to say that Sylvie doing this at the "end of time" is actually concurrent with time passing in general i.e. events in 2012 can still be affected in real time, because that's when Loki's variant was created, it could throw everything after 2012 into complete disarray.

No doubt all of this will be reassembled in some way by Dr. Strange in the Multiverse of Madness, but we're not out of the woods yet when it comes to reality-breaking rules, theories and canon.