Hey there! A little late to the party here, but I work at Mixpanel and we'd be happy to help you out as well :) It's definitely not normal for ~40% of data to be dropping out of your tracking. Like matm above, I wouldn't expect react.js to interfere with Mixpanel tracking in any way, but let's run some tests and see what might be going awry. I'll shoot you an email and we can set up a time to troubleshoot.