grouping timpani and percussion groups


Could anyone explain why Dorico make the timpani and the first percussionist into a group and the second and third percussionist into the other group?

I already changed the groups of the bracket, but I would like to know the reason. Could it be improved in future if it might be regarded as a bug?

“Tam-Tam” belongs to the first percussionist but is grouped with the second, which suggests to me that you’ve already been altering the bracketing yourself. Or are you suggesting that that’s a bug?

Oops, sorry for the ambiguity.
The screenshot had been taken before I changed the grouping. So it is a bug.

What I am expecting from the same setup might be one of the followings: