Mutate not behaving as expected inside custom functions (variation)

Question

My question is similar to Ariel Polani's and Tyler Law's, but in my case the !!sym() solution or the !! solution within mutate() does not work. Here is my code:

library(cansim)
library(tidyverse)
add_series <- function(vector_num,first_date,end_date,tseries) {
  tmp <- get_cansim_vector(vector_num,first_date,end_date)
  tseries <- mutate(tseries,!!sym(vector_num)=tmp$VALUE)
  return(tseries)
}
first_date <- as.Date("2011-01-01",format="%Y-%m-%d")
end_date <- as.Date("2019-06-01",format="%Y-%m-%d")
tseries <- data.frame(REF_DATE=seq(first_date,end_date,by="month"))
tseries <- add_series("v42170711",first_date,end_date,tseries)

Answer 1

If we are using assignment on string, use the := and here we don't need sym

library(dplyr)
tseries <- tseries %>%
                 mutate(!!vector_num :=tmp$VALUE)

-full code

add_series <- function(vector_num,first_date,end_date,tseries) {
  tmp <- get_cansim_vector(vector_num,first_date,end_date)
  tseries %>%
       mutate(!! vector_num :=tmp$VALUE)

}

add_series("v42170711",first_date,end_date,tseries) %>%
     head
#    REF_DATE v42170711
#1 2011-01-01   1566882
#2 2011-02-01   1580492
#3 2011-03-01   1566209
#4 2011-04-01   1594096
#5 2011-05-01   1600677
#6 2011-06-01   1613907