Render multiple data frame with flextable do not show

I have a data frame in R called data and looks like this:

library(tibble)
library(flextable)
library(dplyr)

# Create a tibble
data <- tibble(
  country = c("USA", "Canada", "Germany", "France", "Japan", "USA", "USA", "Germany"),
  project = c("Project A", "Project B", "Project C", "Project D", "Project E", "Project F", "Project G", "Project H"),
  val1 = c(10, 20, 30, 40, 50, 60, 70, 80),
  val2 = c(15, 25, 35, 45, 55, 65, 75, 85),
  val3 = c(12, 22, 32, 42, 52, 62, 72, 82)
)


> data
# A tibble: 8 × 5
  country project    val1  val2  val3
  <chr>   <chr>     <dbl> <dbl> <dbl>
1 USA     Project A    10    15    12
2 Canada  Project B    20    25    22
3 Germany Project C    30    35    32
4 France  Project D    40    45    42
5 Japan   Project E    50    55    52
6 USA     Project F    60    65    62
7 USA     Project G    70    75    72
8 Germany Project H    80    85    82

I want to do some basic calculations and then show in different pages in a pdf each project table.

My attempt is:

flextables_list =  data %>%
  rowwise() %>%
  mutate(mean_val = mean(c_across(val1:val3))) %>%
  group_split(project) %>%
  lapply(function(project_data) {
    flextable::flextable(project_data[, c("project", "country", "mean_val")])
  }) 

and then I print them in order to show them:

for (tbl in flextables_list) {
  print(tbl)
}

But when I render the rmarkdown to PDF, the tables (flextables) do not show. Why?