What happens if you send beam to an expired address? Is it now lost forever?
No, it will stay in your wallet.
It is basicaly like you wanna give cash to someone and you have it in your hand and give him, but he won't take it.
In order to receive that cash , he need to open his hand palm (in this case open his wallet). That's how mimblewimble works (similar as cash in real life). You can't send to someone if he doesn't want to receive.