This problem seems to be OS dependent. If I run the following script I get different results. Locally I'm running on MacOS Mojave use homebrew for PHP 7.2.

Test script:
---------------
<?php
setlocale(LC_ALL, 'C');
var_dump(pathinfo('Ö Ü Å Ø äöüåøhello.txt'));


Output using docker:
---------------
docker run -it --rm --name my-running-script -v "$PWD":/usr/src/myapp -w /usr/src/myapp php:7.2-cli php test-locale-pathinfo.php
array(4) {
  ["dirname"]=>
  string(1) "."
  ["basename"]=>
  string(29) " Ü Å Ø äöüåøhello.txt"
  ["extension"]=>
  string(3) "txt"
  ["filename"]=>
  string(25) " Ü Å Ø äöüåøhello"
}

Output locally:
---------------
php test-locale-pathinfo.php                                                                
array(4) {
  ["dirname"]=>
  string(1) "."
  ["basename"]=>
  string(31) "Ö Ü Å Ø äöüåøhello.txt"
  ["extension"]=>
  string(3) "txt"
  ["filename"]=>
  string(27) "Ö Ü Å Ø äöüåøhello"
}

I assume that you get exactly the same output with basename(), for
which the documentation already states:

| If path contains characters which are invalid for the current
| locale, the behavior of basename() is undefined.

So, not a bug, but needs to be documented for pathinfo() as well.