You can try viewing your Inventory Valuation Detail report using a private browser or incognito browser. This way, we can isolate this unexpected report-behavior for possible a browser-related issue. You can use either of these keyboard shortcuts:
For Google Chrome browser: Ctrl + Shift + N
For Mozilla Firefox browser: Ctrl + Shift + P
For Safari browser: Command + Option + P
Once logged in, you can try generating your inventory valuation detail report again.
On the other hand, you may have a recurring transaction that may have caused the difference with the quantity on hand. Also, you can check for any returned transaction or inventory quantity adjustments that affect the quantity-on-hand of your inventory items.
Lastly, I've added these articles that you can read to help manage your inventory in QuickBooks Online: