Wrong inner borders of merged cells in PDF when bg is specified

[eloimercier-stemcell]

When bg() is specified, the inner borders of individual cells are still displayed even in the merged cells. This only occurs in PDF.
Removing the background colour fixes this issue. fix_border_issues() does not help here.

Reproducible example

library(flextable)
library(officer)
library(dplyr)

data_1 <- data.frame(col1 = c("A1", "A1", "A1"),
col2 = c("B1", "B2", "B3"))
ft <- flextable(data_1)
ft <- ft %>% bg(i=1, bg = "grey90", part = "header") #removing this row fixes the issue
ft <- ft %>% merge_v( j = ~ col1)
ft <- ft %>% border_inner( part="body", border = fp_border(color="red", width = 2) )

print(ft) # the inner borders are displayed properly
print(ft, preview = "pdf") # the inner borders are displayed incorrectly in the merged cells