Let's perform some troubleshooting steps to isolate this. I recommend using a private or incognito window and create an invoice for that client. This way, we can check if the issue you're having is caused by stored cache from the browser.
Stored cache for some time can cause unusual behavior that affects the performance of your QuickBooks Online (QBO) account. To start with, here are the keyboard shortcuts in opening a private/incognito window:
Google Chrome: Ctrl + Shift + N
Internet Explorer & Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
If you're able to create an invoice for your client while in a private or incognito window, then we'll need to clear your regular browser cache. This will erase the cache stored from the browser and prevents unusual behavior
However, if the issue persists, I suggest using another supported browser. This way, we can check if this is a browser integrated issue.