I'd be delighted to assist you and ensure you'll be able to see the Customer/Project field again from the Bill screen.
The first thing we can do is ensure that Track expenses and items by customer is turned on from the settings. Here's how:
Go to Settings ⚙, then select Account and settings.
Go to the Expenses tab.
From the Bills and expenses section, select Edit ✎.
Turn on the Track expenses and items by customer.
If the issue persists, try to log in to your QBO account using a private window. There are times when too much historical data from the browser causes discrepancies that affect the view and performance of your QuickBooks. Thus, you're unable to see the field. Feel free to use either of these keyboard shortcuts to open an incognito window, depending on your browser:
Google Chrome: Ctrl + Shift + N
Safari 11 or newer: ⌘ + Shift + N
Mozilla Firefox: Shift + Ctrl + P
Microsoft Edge: Ctrl + Shift + N
If it works, you can return to your original browser and clear its cache to start fresh. You can also use another supported, up-to-date browser that is already installed to narrow down the results.