It seems that NA_character_ is handled as a string of length 2 when writing a SAS transport file with write_xpt(). Consider for example:

t <- data.frame(MYFL = c("Y", NA_character_), id = c(1, 2))
attr(t[["MYFL"]], "width") <- 1
haven::write_xpt(t, "t.xpt", version = 5, name = "t")

It produces

Warning message:
Column `MYFL` contains string values longer than user width 1. Width set to 2 to accommodate. 

and the length of MYFL in the SAS transport file is 2.

Is this intended?

I would expect length 1 and no warning.