My email list split test crashed hard last Tuesday

I set up an A/B test for a welcome email series using Mailchimp, trying two different subject lines. The plan was to send to 5,000 new subscribers over 48 hours. The system glitched and sent the losing variant to the entire list after just 100 opens. I had to pause the whole campaign and send a correction email, which looked super unprofessional. On one side, you just eat the loss and move on. On the other, you try to salvage data from the messed up send. Has anyone else had a platform fail like this and how did you handle the data?