You still have to pay a tip for a cancelled service.

Happens with other services - like medics and prostitutes.
I went away from that mechanic and as I got far enough for the service to be canceled, I was offered to give a tip.

0:00 - 0:20 in the video:



https://youtu.be/FiJruSp39hc
Hi, You were probably had connection issues such as lags, high packet loss. Because usually the tip window pops up instantly after the service window screen disappears. I don't think it's a bug. Plus you can cancle the tip by clicking "close" in the Tip window that way you don't have to give any tip.
I am aware of this being an issue it’s just not a massive issue right now.