Changing $0 after gsub() breaks output

[Yasuhiro Yamada]

Hi.
Executing gsub() without an action and making changes to $0 will
corrupt the output.
This behavior seems like a bug.

    $ ./gawk --version
    GNU Awk 5.2.1, API 3.2, PMA Avon 8-g1
    ...
    $ echo abc | ./gawk 'gsub(".","@") && $0=$1'
    q  <===== broken output

Interestingly, the result is different for each run.

    $ echo abc | ./gawk 'gsub(".","@") && $0=$1' | od -tx1c
    0000000  b0  61  99  0a
             260   a 231  \n
    0000004
    $ echo abc | ./gawk 'gsub(".","@") && $0=$1' | od -tx1c
    0000000  c0  e7  f7  0a
            300 347 367  \n
    0000004
    $ echo abc | ./gawk 'gsub(".","@") && $0=$1' | od -tx1c
    0000000  c0  07  14  0a
            300  \a 024  \n
    0000004
    $ echo abc | ./gawk 'gsub(".","@") && $0=$1' | od -tx1c
    0000000  c0  57  76  0a
            300   W   v  \n
    0000004

Older versions do NOT reproduce this issue.
Also, the outputs are as expected and intuitive.

    $ ./gawk --version
    GNU Awk 4.1.4, API: 1.1
    ...
    $ echo abc | ./gawk 'gsub(".","@") && $0=$1'
    @@@
    $ echo abc | ./gawk 'gsub(".","@") && $0=$1' | od -tx1c
    0000000  40  40  40  0a
              @   @   @  \n
    0000004

This issue seems to occur in v4.2.0 and later.

    $ ./gawk --version
    GNU Awk 4.2.0, API: 2.0
    ...
    $ echo abc | ./gawk 'gsub(".","@") && $0=$1' | od -tx1c
    0000000  e0  4f  fc  0a
            340   O 374  \n
    0000004

My environment is

    $ uname -a
    Linux ip-172-31-9-222 5.4.0-1093-aws #102~18.04.2-Ubuntu SMP Wed
Dec 7 00:31:59 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
    $ gcc --version
    gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0