I think this is fixed now! Letters should be immediately marked as placed even when the animation is still going on, so sending the second letter will put the first letter back in the top column rather than overwriting it.
I'm not exactly sure how you managed to run into this - to test it I had to slow the animation down to give myself time to click, haha - but please let me know if you're still seeing it after the fix. Thanks again for the bug report!