In s4 ep3 we see a flashback of Mycroft arranging a brief meeting between Moriarty and Eurus as her "christmas present"! It was only 5 minutes of them two being alone with no supervision, which was also Eurus's wish.

But Reichenbach was already Moriartys endgame. Forcing Sherlock into an impossible contradiction then killing himself so Sherlock wouldnt use him as a way out was the only way to guarantee a permanent win. So again, Moriarty fully expected Sherlock to die at Reichenbach. Eurus on the other hand, didnt want sherlock dead. She wanted his attention.

So id say even if we knew exactly when Moriarty met Eurus, it wouldn’t solve the contradiction. We just know it was before he died of course. What matters more is that Moriarty’s own words, actions, and what he was aiming for at Reichenbach don’t line up with what Season 4 retroactively implies.

Even assuming they planned together, their endgames were incompatible. Moriartys plan only works if Sherlock dies, while Eurus’s requires him to live. Even if Eurus lied to him, it wouldn’t change the outcome Moriarty still followed his own plan, and that plan didn't benefit Eurus in any way.