inconsistent output for field reference for a number stored as a string

[Ed Morton]

Using:

   $ gawk --version
   GNU Awk 5.0.1, API: 2.0 (GNU MPFR 4.0.2, GNU MP 6.2.0)

I'd have expected all of these to produce the same output:

   $ echo 'a b c' | awk '{print $("2")}'
   b
   $ echo 'a b c' | awk '{print ($"2")}'
   b
   $ echo 'a b c' | awk '{print $("2")}'
   b
   $ echo 'a b c' | awk '{printf "%s\n", $"2"}'
   b
   $ echo 'a b c' | awk '{a=$"2"; print a}'
   b
   $ echo 'a b c' | awk '{print $"2"}'
   a b c

but as you can see that last one doesn't and I don't know why. Is it a 
bug or, if not, what's the explanation?

    Ed.