Well, IMHO, it is not a taxable event because it's just a multisig peg between btc and lbtc. Traditional alt-coins are not pegged like this. Of course the IRS would consider anything they can't trace with chain analysis from the original buy to the final sale as a taxable event.

It's more about, don't get audited rather than don't tell them. Easier said than done as you have no control over this.