Plant cells need water to stay rigid. If they become too dry the spaces in between cells collapse because they are empty space causing sag.

That's the only thing I can think of at the moment.

Perhaps you may have severed the taproot in the transplant. If so you will know by tomorrow for sure. In that event it will shrivel and die rather quickly.

Don't beat yourself up over it tho, it's only a 7 day loss, and you can always pop more beans.