I already reported this issue last summer (unfortunately I cannot find a link to it) and it was supposedly fixed - when doing bulk inserts of many records, the data given to Piwik is not reset after every visit entry for custom variables with scope "visit". I tested it again with the newest release just now, and the same bug still exists:

In my case, I am inserting 1000 page visits at a time. As soon as a custom variable with scope "visit" is set, all further visits seem to be added to this first visit, even though I am setting an explicit visitor ID (via setVisitorId in PiwikTracker class) for every entry.

This leads to HUGE visitor log entries - suddenly one visitor seems to have visited 500 pages and made 4 purchases within his journey (which would be great, if anyone would ever do that).

Currently, to circumvent this problem, every time a custom variable with "visitor" scope is set I issue a $t->doBulkTrack(); and start again. Unfortunately, this is not very efficient - instead of 1000 entries at a time, there are many requests with a small number of inserts. So a fix would be greatly appreciated.