<?php
xdebug_start_code_coverage();
$c = [
$a ?? null,
$b ?? null
];
var_dump(xdebug_get_code_coverage());

Should return 3 lines 4,5 and 7 currently misses libe 5 ($b ?? null)

Changing the line to $b ?? 0 "fixes" the issue. ie the below returns as expected

<?php
xdebug_start_code_coverage();
$c = [
$a ?? null,
$b ?? 0
];
var_dump(xdebug_get_code_coverage());