It's not a bug. Typically, bush combos have a "Bush->Next" combo flag associated with them, meaning when Link slashes the plant, the combo transforms to the combo directly after it on the combo list.
Make sure that the bush combo has the "Bush->Next" combo flag and make sure that the combo showing the slashed plant is directly after it.
This is the easiest fix. Depending on how you want to set up your bushes, you can also set the screen's under combo to be a slashed bush, but this is an old-school method and not recommended in 2.50.
Veterans, please correct me if I'm wrong here. It's been a while since I've played with these things.