Some wikis, e.g. Hungarian Wikipedia (see T14085), have a patroller user group and bureaucrats on those projects can place users in that group and remove them from it. However, stewards aren't able to do the same. Even though it is quite unlikely that stewards would need to change that group, there should be a patroller group (if nothing, then at least for consistency's sake) in the Userrights interface for stewards when dealing with wikis that do have that user group. This shouldn't be too hard to fix, I imagine.
Additionally, if the user group exists on the local wiki but not the target wiki, the user is added to a group that doesn't really exist