Steps to reproduce the issue

1. fresh installation
2. new article
3. insert read more
4. insert page break
5. navigate to article
6. click link to page break page in index

Expected result

page shows up

Actual result

404 error

System information (as much as possible)

Litespeed 5.2.1
php 7.0.23
5.5.5-10.1.26-MariaDB
CentOS 7.3 (with Cpanel 66.0.23)
Safe Mode Off
Open basedir /home/user:/usr/lib/php:/usr/php4/lib/php:/usr/local/lib/php:/usr/local/php4/lib/php:/tmp
Display Errors On
Short Open Tags On
File Uploads On
Magic Quotes Off
Register Globals Off
Output Buffering Off
Session Save Path /var/cpanel/php/sessions/ea3
Session Auto Start 0
XML Enabled Yes
Zlib Enabled Yes
Native ZIP Enabled Yes
Disabled Functions None
Multibyte String (mbstring) Enabled Yes
Iconv Available Yes
Mcrypt Enabled Yes
Maximum Input Variables 1000

Additional comments

Page break links work without read more, but not with read more (hr id="system-readmore"). Tested on a new clean Joomla 3.7.5 upgraded to 3.8 and on clean 3.8 installation.
Can observe the difference here:
http://gofardesign.uk/joomlatest/index.php/2-uncategorised/2-test-article-2 (without system-readmore)
http://gofardesign.uk/joomlatest/index.php/2-uncategorised/1-test-article (with system-readmore, page link not working)

Tested with and without SEO urls