When sliding, UI copies stone rather than moving it

[blahedo]

When the user slides a piece, the UI acts as if that piece has been copied rather than moved. In this screenshot, the black player has selected the stone at C7 to be moved:

And after clicking on F7 to slide it there, a black stone is drawn at both C7 and F7:

The bot console output reports the move correctly:

Player black slides a piece from C7 to F7.

And on page reload, the board is drawn (correctly) with a black stone at F7 but not C7:

This is game # 5797941e3a0ac6306cd86c11

[blahedo]

I should add that this often happens, I think even most of the time that I slide a piece, but it's not 100% of the time.