Is Adobe Analytics implemented with first party cookies?
option 1: If it isn't, you could try to break down the mcvids with browser to see if it's related to safari browsers only. If this is not purely safari browsers but all over the place, then it might be worth looking at this option:
option 2: I have seen issues similar to your one which was not related to any firstparty cookie issue, here we saw that when users clicked through from one domain to another subdomain, the mcvid would sometimes get reset (was on several calls with the adobe engineering team, which didn't understand what was going on either), our workaround was to append the visitorid to the url and then have the code on the subdomain pick it up.
p.s the above are just some ideas for you to go further with
Either way, I would probably look into option1 regarding first party cookies (everything you need is documented here: First-party cookies )