Let's make sure you have the customer or class field after adding or matching a transaction.
I've tried adding and matching a bank transaction in QuickBooks Online, and it seems to have worked just fine. The customer and class fields are showing.
This might be a web-page cache issue since the customer and class fields disappeared after adding or matching a transaction. To verify this, we can try using your browser's incognito feature. This feature doesn't use the existing cache files, which makes it a good place to test if there's an issue with the browser.
Press the Ctrl + Shift + N (Google Chrome) keys on your keyboard.
Press the Ctrl + Shift + P (Firefox or Internet Explorer) keys on your keyboard.
Press Control + Option + P (Safari) keys on your keyboard.
If the customer or class fields show when using the incognito feature, I suggest you go back to your main browser and then clear the cache. This step resolves most browser-related issues. Alternatively, you can use a different browser (Internet Explorer, Chrome or Mozilla).
If it's doing the same thing, I have a workaround for you. You can locate the transaction from the Bank Register and add the customer on it.
Select the Gear icon and choose Chart of Accounts.
Look for the bank account and click Register.
Locate and click the transaction.
Click the Edit button, then update the customer or class.
Select Save and close.
Please let me know how this turns out. I'm right here if you need more assistance. Enjoy your day.