Current behavior as documented in https://piwik.org/faq/how-to/faq_19616/ from #9299 :

By default Piwik will create a new visit whenever a visitor views your website using a different campaign information from the visitor’s ongoing visit. You can disable this behavior by setting 1create_new_visit_when_campaign_changes=0under[Tracker]` section in your config/config.ini.php file.

Proposal: set by default create_new_visit_when_campaign_changes=0 in global.ini.php

Goal: this will prevent some possible confusion when many visits are created if links happen to include Piwik campaign parameters. By default, Piwik should be careful to create only one visit within 30 min period.