That’s the whole point of symlink:, I think:
https://docs.platformio.org/en/latest/core/userguide/pkg/cmd_install.html?highlight=symlink#local-folder
The file: version fetches a local copy and never looks back, just as it does for other URLs referring to remote libraries. The symlink: model avoids copying and continues to track changes, so that the project will rebuild if the library changes. It’s super convenient for a range of usage scenarios, IMO.
See also Migrating from 5.x to 6.0 — PlatformIO latest documentation.