R formattable negative number formatting (using currency function)

Question

I am fairly new to formattable and am trying to output a R Markdown document with different sales figures. I am preparing the figures and then display them using formattable.

I use the following code to create and format the table. The q_line input is a data.frame containing employee names in the first column and numbers in the following columns, example data as follows:

Column A	Column B
Jane	-10000
John	+5000

formattable(q_line,
            align = c("l", rep("r", ncol(q_line)-1)), list(
                area(, col = 2:ncol(q_line)) ~ num_format,
                area(row = nrow(q_line)) ~ last_row
            )
)

I also define the num_format format as follows:

num_format <- function(x) {
  x <- currency(x, symbol = "CHF", sep = " ", digits = 0L, format = "d", big.mark= "'")
}

When I enter a negative number (e.g. -10000) to num_format, the result is as expected (i.e. returning "CHF -10'000"). However, in the formattable output, the number shows as a positive, i.e. "CHF 10'000".

I tried to find where the "-" is dropped by not applying the num_format to the formattable (worked), by using the num_format function outside formattable (worked) and using accounting instead of currency function for formattable (worked, but the result is not satisfactory).

Any ideas why the "-" is not displayed?