Wrong (double) border when saving as image.

Hello together,
I have never posted to a forum, but now I've got a problem, that I can't solve.
I create a table with flextable and add a header, that combines two columns. When I save the table as a png.file, the right border in the combined column is much sicker as the others.

Here is my code:

library(flextable)

df <- data.frame(a = c(76,80,30,40),
                 b = c(34,46,56,67),
                 c = c(45,45,67,34),
                 d = c(34,3,44,55))
header1 <- c("one","two","three")
df %>% flextable %>%
  add_header_row(colwidths = c(1,2,1), values = header1) %>%
  align(align = "center", part = "all") %>%
  hline(i = 1, part = 'header', border = fp_border_default(width = 0)) %>%
  vline(j = c(1,3), part = "all") %>%
  save_as_image("test.png")

It looks like this:
image

Perhaps there is anyone who can help me?