I’ve always thought it works perfectly. No one in-story has any reason at all to expect it, so neither should we. It’s a genuine surprise to everyone - characters as well as audience.

Plus, there is a sort of set-up (whether Lucas intended it or not). Han is the only one who knows what he’s doing, not even Chewie knows. He may be giving us a hint when he says “I know what I’m doing”. To me, it feels like he may already have it in his mind to consider helping them.

However, true to his character he first secures Chewie, their pay, and their ship. He gets what’s important to him out of harm’s way and then makes the final decision to help from a position of relative safety - off of the target planet & base, away from the notice of the Empire.

No issues with that scene at all for me.