Parts of this actually also seem broken on earlier versions, for example:

preg_match('/(?J)(?:(?<g>foo)|(?<g>bar))(?<h>baz)/', 'foobaz', $matches);
var_dump($matches);

array(6) {
  [0]=>
  string(6) "foobaz"
  ["g"]=>
  string(0) ""
  [1]=>
  string(3) "foo"
  [2]=>
  string(0) ""
  ["h"]=>
  string(3) "baz"
  [3]=>
  string(3) "baz"
}

Key "g" should be "foo" here though.

Looks like the actual problem existed in 7.3 and earlier, but it required slight modification to be reproduced. I'll try to update the bug report to take it into account.

This actually happens on all versions, if we consider the following example

<?php
preg_match('/(?J)(?:(?<g>foo)|(?<g>bar))(geez)/', 'foogeez', $matches, PREG_UNMATCHED_AS_NULL);
var_dump($matches);
?>

Expected result:
----------------
array(5) {
  [0]=>
  string(7) "foogeez"
  ["g"]=>
  string(3) "foo"
  [1]=>
  string(3) "foo"
  [2]=>
  NULL
  [3]=>
  string(4) "geez"
}

Actual result:
--------------
array(5) {
  [0]=>
  string(7) "foogeez"
  ["g"]=>
  string(3) "foo"
  [1]=>
  string(3) "foo"
  [2]=>
  NULL
  [3]=>
  string(4) "geez"
}

I've fixed this in PHP-7.4. I don't want to fix this on 7.3. It's a long standing issue, and changes to PCRE matches output are risky.