A/B Testing stopped working

I currently have the A/B Theme testing activated on a large number of my clients sites. They are all running the exact same installation, same themes, same plugins, same everything. I'm using this one as an example. Seems all tracking for the split of the themes in the Custom Variables section of the reporting has stopped ever since the 19th of October. I have no clue why especially since everything has been running smoothly. But this is an issue on all my clients sites. Could someone check on this for me to see what the issue could be? Thank you!