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 few a page title report, then all pages with a title that contained example used to be shown. With this new behaviour, all page titles would be shown that were viewed by visitors who also viewed a page with the title example as part of their visit. This means it would 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 previous behaviour, you can edit your config/config.ini.php and configure the following setting:

[General]
enable_segments_cache = 0