Why can internal UTM links change GA4 attribution even when they do not start a new session?

Because GA4 treats session attribution and key-event attribution differently. The original landing hit's `session_start` sets the session source/medium, so clicking an internal link with UTMs usually doesn't create a second session or replace that session's source. But those UTMs can still be read as a new campaign touchpoint for later key events, so a conversion path or event-scoped Source/Medium report can give your own `internal` campaign credit. Google even shows a session that started from paid search, then hits a UTM-tagged link mid-session; the session stays paid search while the later key event path includes the tagged source.

So don't put `utm_source`, `utm_medium`, or `utm_campaign` on normal site navigation. Use internal event tracking instead—like a `select_promotion`, CTA-click, or custom event with placement details—then keep UTMs for links that actually bring someone onto the site from elsewhere.