How to make gtsave do not ignore borderlines?

Question

I need to create a table in gt. In my code which I run through output you can see dotted lines.

But in file saved using gtsave there are no such lines.

Why and to change it? Lines where produced

tab_style(
style = cell_borders(
  sides = c("bottom"),
  color = "gray",
  weight = px(1.5),
  style = "dotted"
),
locations = cells_body(rows = c(3, 6, 9, 12, 15, 18, 21))) 

And to save it:

gtsave(tab1, "tab1.png")