Yes, this is a bug, thanks for letting me know. I will be doing a bug-fixing patch soon, to get that issue and some others I have noted down.
There is a second way to defeat the miniboss though, which will work in the meantime. If you do a regular bomb explosion when the turtle is directly on top of the bomb, that will work as well. (The idea is you hit the weak underside). That should let you finish the trial.