Instance Variable Available in Single Test, but not in full test suite

My test:

def setup
    @seminar = seminars(:one)
end

test "Auto user_number" do
    debugger
    post students_path, params: { students: [{ first_name: "Noob",
                                last_name: "Sauce" }],
                                ss: { seminar_id: @seminar.id } }
    student = assigns(:student)
    assert_equal student.id, student.user_number
end

When I run this test file alone, everything passes as expected. But when I run the full test suite, it breaks on @seminar.id.

When I pause at the debugger and call @seminar, the result is nil.

I've already checked the seminar fixture for seminars(:one). Also, this same fixture is working fine in a dozen other test files.

Thanks in advance for any insight.