Haven't run into the bug you describe yet I feel this is similar as your rules do not clarify against this, but I'm not able to use the two corners on the same block here(having to also back out of selection and reselect on the right corner to able to send it to the right). https://my.mixtape.moe/kmqjbw.webm
Other than that this seems like a fun little game especially in multiplayer and there's a few ways you could perhaps improve it by adding a small replay-like feature to see the previous player's move. It's very easy to lose track of a singular move in this game of blendable lines and as a replay example, Lichess' replay function is great. Leaderboards and rankings on the multiplayer side along with colour selections and just a mode with a timer for singleplayer ala speed chess.