Thanks for reaching out to the Community for support and sharing your concerns. I'm happy to lend a hand.
I recommend we try some basic troubleshooting steps, such as clearing the cache from your web browser. Web browsers collect cache to save time when loading repetitive data and images. Over time, these files can become outdated and corrupted, causing issues like adding your new sub-customer. I've included links for all supported browsers that contain the steps to clear the cache below.