scale_fill_manual not respecting to the order of values and labels

Question

For the code below, I try to use scale_fill_manual('trend', values = c('red', 'blue', 'gray'), labels = c('up', 'down', 'keep')) to set bar color for each month with red for up, blue for down and gray for keep:

df <- structure(list(month = structure(c(1L, 1L, 2L, 2L, 3L, 3L, 4L, 
4L, 5L, 5L, 6L, 6L, 7L, 7L, 8L, 8L, 9L, 9L, 9L, 10L, 10L, 11L, 
11L, 12L, 12L), .Label = c("1", "2", "3", "4", "5", "6", "7", 
"8", "9", "10", "11", "12"), class = "factor"), trend = c("down", 
"up", "down", "up", "down", "up", "down", "up", "down", "up", 
"down", "up", "down", "up", "down", "up", "down", "keep", "up", 
"down", "up", "down", "up", "down", "up"), numbers = c(10L, 16L, 
16L, 11L, 15L, 12L, 14L, 13L, 15L, 12L, 15L, 12L, 14L, 13L, 16L, 
11L, 14L, 1L, 12L, 15L, 12L, 14L, 13L, 15L, 12L)), class = "data.frame", row.names = c(NA, 
-25L))

ggplot(df, aes(x = month, y = numbers)) +
  geom_bar(aes(fill = trend), alpha=0.8,
           stat='identity', position = 'dodge', width = 0.7) +
  scale_fill_manual('trend', values = c('red', 'blue', 'gray'),
                    labels = c('up', 'down', 'keep')) +
  geom_text(aes(x = month, y = numbers, label=numbers, group=trend), position = position_dodge(width=0.9), vjust=-0.25) +
  labs(x='month', 
       y='numbers') +
  theme(
    legend.position='bottom', 
    legend.title = element_blank()
  )

Now the problem we can find in this figure is: red for down, blue for keep and gray for up, which is different from the requested.

If we comment scale_fill_manual(...) part and rerun the code, the default order correspondence is correct:

So I'm wondering why I get this issue and what's the correct method to set customized colors for this specific data (only one keep entry for September group)? Thanks.

Data:

   month trend numbers
1      1  down      10
2      1    up      16
3      2  down      16
4      2    up      11
5      3  down      15
6      3    up      12
7      4  down      14
8      4    up      13
9      5  down      15
10     5    up      12
11     6  down      15
12     6    up      12
13     7  down      14
14     7    up      13
15     8  down      16
16     8    up      11
17     9  down      14
18     9  keep       1
19     9    up      12
20    10  down      15
21    10    up      12
22    11  down      14
23    11    up      13
24    12  down      15
25    12    up      12