Steps to reproduce the issue

Tested on PHP 7.2.3
You need a unix based server.
Install Joomla in the root path: /
Absolute paths:
/index.php
/administrator/
etc...
Ps.: I know it is strange, but I think there are special context when this can happen.

Expected result

JPATH contants should contain a path without the ending directory separator

Actual result

JPATH constants contains the directory separator

Additional comments

In this environment __DIR__ constant holds / in the root folder. Normally the __DIR__ constant holds the path without the ending directory separator like /public_html

includes/defines.php

// Global definitions
$parts = explode(DIRECTORY_SEPARATOR, JPATH_BASE);

// Defines.
define('JPATH_ROOT',          implode(DIRECTORY_SEPARATOR, $parts));
define('JPATH_SITE',          JPATH_ROOT);
define('JPATH_CONFIGURATION', JPATH_ROOT);

So all of this constants will be / instead of empty string.

Possible solution:

Probably __DIR__ holds the not expected value which should be fixed in index.php

define('JPATH_BASE', __DIR__ === DIRECTORY_SEPARATOR ? '' : __DIR__);

Is it a __DIR__ bug in PHP itself?

I do not think so, PHP describes __DIR__ constant with the following:

The directory of the file. If used inside an include, the directory of the included file is returned. This is equivalent to dirname(FILE). This directory name does not have a trailing slash unless it is the root directory.

http://php.net/manual/en/language.constants.predefined.php