dplyr Group - Do I need to then Ungroup - r

Question

Here's some simple code utilizing dplyr to group and spread data from the mtcars data set.
library(dplyr)
mtcars.df <- mtcars %>%
group_by(disp, cyl) %>%
summarise(Qty = n())
mtcars.spread <- mtcars.df %>%
spread(cyl, Qty)
str(mtcars.spread)
When you look at the structure of the 'mtcars.spread' tibble you'll notice the '4' and '6' cylinder variable are listed as integers, while the '8' cylinder variable has all this babble
attr(*, "vars")= chr "disp"
attr(*, "drop")= logi TRUE
attr(*, "indices")=List of 27
attached to it. Where did I go wrong? Am I supposed to ungroup along the way after using the group_by command?
Classes ‘grouped_df’, ‘tbl_df’, ‘tbl’ and 'data.frame': 27 obs. of 4 variables:
$ disp: num 71.1 75.7 78.7 79 95.1 ...
$ 4 : int 1 1 1 1 1 1 1 1 1 1 ...
$ 6 : int NA NA NA NA NA NA NA NA NA NA ...
$ 8 : int NA NA NA NA NA NA NA NA NA NA ...
- attr(*, "vars")= chr "disp"
- attr(*, "drop")= logi TRUE
- attr(*, "indices")=List of 27
..$ : int 0
..$ : int 1
..$ : int 2
..$ : int 3
..$ : int 4
..$ : int 5
..$ : int 6
..$ : int 7
..$ : int 8
..$ : int 9
..$ : int 10
..$ : int 11
..$ : int 12
..$ : int 13
..$ : int 14
..$ : int 15
..$ : int 16
..$ : int 17
..$ : int 18
..$ : int 19
..$ : int 20
..$ : int 21
..$ : int 22
..$ : int 23
..$ : int 24
..$ : int 25
..$ : int 26
- attr(*, "group_sizes")= int 1 1 1 1 1 1 1 1 1 1 ...
- attr(*, "biggest_group_size")= int 1
- attr(*, "labels")='data.frame': 27 obs. of 1 variable:
..$ disp: num 71.1 75.7 78.7 79 95.1 ...
..- attr(*, "vars")= chr "disp"
..- attr(*, "drop")= logi TRUE