Since Matomo 3.12.0 the segments behave no longer like a filter but instead include all data of visits that match a certain segment.

This means if you have a segment like page url contains example, and you view a Page title report, then all pages with a title that contained example used to be shown (before Matomo 3.12.0). With this new behaviour (3.12.0 and newer), all page titles are now shown that were viewed by visitors who also viewed a page with the title example as part of their visit. This means it does not only show pages that contained the word example, but also all the other page titles that were viewed by these matching visits.

To restore the previous behaviour, you can edit your config/config.ini.php and add the following setting:

[General]
enable_segments_cache = 0