It's not a bug but a suggestion, you know how his combo jumps to enemy after counter triggers, if the target enemy servant died while FA is doing the 7 cuts, he will still follows the tsubame gaeshi skill, but without a target and stay on the same spot, stuck until the combo ends.

I would suggest make a trigger of valid/invalid targeting before the tsubame gaeshi triggers so this would not happen, as hitting air even after the target died before it is alittle bit dumb.

Same goes for Lancer's (cu chulain) combo. It still stabs if the target dies killed by his teammates before finish chanting. (This also makes him able to kill invurnable target like red saber when she is in the middle of reviving)

He is also able travel pass 9999 distance after activated the skill and chase the target until it hits if the target transported into either ubw/IH or vice versa, or after the target teleported away with the scroll.