You're able to manage your customers/clients list by editing their information through the Customers menu in QuickBooks Online (QBO). Then, a customer profile won't be gone on your list unless it's marked as inactive.
There are times that the browser is full of frequently accessed pages, thus causing some unusual responses to the system. You can isolate this by logging into your QuickBooks Online (QBO) account using a private browser (incognito).
Press Ctrl + Shift + N (Google Chrome)
Ctrl + Shift + P (Firefox)
Control + Option + P (Safari)
Once signed in, go to the Sales menu, then select the Customers tab. If you're able to view the complete list of your clients, return to your default browser and clear its cache and cookies. This is to remove older data that causes viewing and performance issues. However, if the problem continues, I'd recommend using other supported browsers.
In the meantime, I'm adding this article to effectively monitor your sales, income, and customer transactions in QBO: Manage customers and income. It includes topics about invoice payments, customer refunds, and customer lists to name a few.
Please let me know how it goes. If you have other concerns or follow-up inquiries about managing your customer list in QBO, I'm just around to help. Take care always.