Plugins are pinned after a stable release. That means you won’t automatically get newer plugin updates that might rely on changes in the Discourse core which aren’t part of the stable release yet.
If a plugin update uses code that only exists in the latest tests-passed branch, it could break on stable because that code simply isn’t there.
For example the AI plugin started using the js-diff library, which was only added to core Discourse after the February stable release. So the latest version of the AI plugin won’t work with that stable version.