I had similar issues. It seems that sometimes appendVisitorIDsTo() does not apped the MCID. The solution for me was to call first getMarketingCloudVisitorID() and run appendVisitorIDsTo() only after that.
Hi Alexis Cazes Adobe - I tried using appendVisitorIdsTo() but did not have any luck. The correct MCID is not passing through on Safari. I even included it on a specific link such as the example here suggests and had the query parameter present upon page load at the second domain. The MCID was not successfully overwritten with what was in the query parameter.
Any other solutions or idea why this would happen?