Iran seems to believe that their negotiating position will get better and better as time goes on. I'm not sure if they're right, but the terms they got Trump to agree to certainly indicate that they are. From that perspective, their current position regarding Lebanon makes sense. The more Israel attacks Lebanon, the more concessions Iran is able to extract from the US in their negotiations in exchange for not resuming the war. It's clear at this point that the only realistic outcome of the war is a negotiated settlement with Iran, so all resuming the fighting would do is kick the can down the road to a point where Iran's position is even better and the US's position is even worse.