Did you change/add the expericence cloud visitor ID service? you can check if you see an "AMCV_xxx@AdobeOrg" cookie, see here: Analytics Cookies
As far as I know when you use this ID service, it will set/use the "mid" and no s_vi is set at all.
easiest way is change your eVar to something like
eVar64="D=mid" to capture the new ID.
or you can create a data element which tries to get both values (combined with a delimeter) to capture old and new ID.