Many, many thanks! Your answer pointed me towards the right direction! First of all, it was weird that when column names were quoted, it worked on your machine. This was the sign that I had to update my packages. Afterwards, I re-read the doc on defusing and changed the do.call call from:
do.call(.f, list(...)),
to
do.call(.f, eval(substitute(alist(...))))
doing so now works:
purely(select)(mtcars, mpg, am, starts_with("c"))
$result
data.frame [32, 4]
mpg dbl 21 21 22.8 21.4 18.7 18.1
am dbl 1 1 1 0 0 0
cyl dbl 6 6 4 6 8 6
carb dbl 4 4 1 1 2 1
$log
NULL
Thank you!