What bounce rate means in GA4
GA4 bounce rate is the share of sessions that are not engaged. An engaged session passes the configured engagement-time threshold, contains a key event, or has at least two page or screen views. The default time threshold is 10 seconds; check your property’s setting.
This is not the old single-page-session definition. Changes to key events or engagement settings can alter the metric without changing visitor behaviour. Investigate measurement integrity if the shift coincides with a tracking release.