Once a browser has been asigned a global visitorid (aka _pk_uid) site-local overrides should not change its value.
Note: The Queued Tracking Plugin also needs to be adjusted for this change.
At the moment this is just a proposal, I will contact @mattab to discuss this issue.
FYI: I've re-created the PR after changes in https://github.com/matomo-org/matomo/pull/12755/files
And also created: https://github.com/matomo-org/plugin-QueuedTracking/pull/74