It's telling you that it had a power failure, not that it's not bound.

It's not smart enough to know that the power failure was caused by you changing the battery. If the reciever powers up and sees the transmitter on the last two channels it used it assumes the power went off and flashes the lights.

It works perfectly well with the lights flashing though.

As Jim said, the way to get the transmitter to pick two new channels is to turn it off and on again.