> It looks like calling unset() on one array element and then appending an item to this array produce a different
> result in PHP 7.2+.
Close. PHP won't reuse the removed key in any version, even if it was at the end of the array.
The problem here is in array_values: https://3v4l.org/vlTD7

> $old = ['one', 'two', 'three', 'four'];
> unset($old[3]);
Before and after, $old is a particular form of array where it has consecutive integer keys starting from 0 in ascending order ("packed without holes"). In other words,
  array_keys($old) === range(0, count($old) - 1)

https://github.com/php/php-src/blob/php-7.2.0RC4/ext/standard/array.c#L4016
In PHP 7.2 array_values() gained an optimization: for packed no-hole arrays it returns the array as-is because the reindexing would not have changed anything, however it would have reset the value for the next array key by virtue of creating a new array. So it looks like there should be a check added for that as well. (Or maybe it gets bundled into HT_IS_WITHOUT_HOLES?)

https://github.com/php/php-src/commit/cc96166f743011c037d9915681b28c363c189e0a should address this (thanks for the catch during pre-release!)

Going to keep this open for a bit as array_slice() is impacted by a similar root cause (in 7.3 only) and this is as good a place to track it as any.

It looks like this regression still affects array_values on empty arrays (arrays which previously had values but values are all unseted before using array_values):
https://3v4l.org/U4qRd

As domen at jollydeck dot com pointed out, the issue is not solved for empty arrays. My test case is so close to his it seems copy-and-paste'd : https://3v4l.org/5rncA

Issue is fixed as of PHP 7.2.2.