gensub() matched component always empty, even for the exact text of the example in the manual

[Zoli Márk]

Hello -
I have run into a problem for which I needed the (g)awk function gensub(), 
namely the "identify matched components and reuse them in the replacement text" 
kind of problem. However, it seems that the component thus matched is always 
empty, and investigating the issue has shown that in this particular gawk not 
even the manual example works. 
This happens on Ubuntu 20.04. 
gawk -V replies "GNU Awk 5.0.1, API: 2.0 (GNU MPFR 4.0.2, GNU MP 6.2.0)" plus 
boilerplate. I can't tell anymore if I installed it from some repository or it 
came with Ubuntu - I am, however, sure that I did not install it from sources.
The manual I'm referring to is the one linked from the 
https://www.gnu.org/software/gawk/ page, namely 
https://www.gnu.org/software/gawk/manual/gawk.html 
The example is the following:
$ gawk '
> BEGIN {
>      a = "abc def"
>      b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a)
>      print b
> }'
-| def abc
Instead of printing the promised "def abc", on my computer this code prints an 
empty line only, as both \\1 and \\2 are empty. (Actually it's not an empty 
line - it contains the space from between \\2 and \\1 in the replacement text, 
as it did with other characters in my code; only the matched components 
themselves are empty.) Is this a bug?
Best regards 
       M- S- Z-