It could be that accumulated temporary internet files are the root cause of this unusual behavior.
To fix this, you can open your account using a private browser (incognito). This session doesn't store cache or cookies in the system. Thus, a good place to fix issues like this.
You can use these keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
If customers now show up on the invoice, I suggest going back to your original browser and clearing the cache. As an alternative, you can also use another supported browser.