I can help you rectify this unexpected behavior and create an inventory adjustment successfully in QuickBooks Online (QBO), @acctggst. 

One possible cause of the behavior you're experiencing is the result of outdated and corrupted cache files in your web browser. While browser caches help speed up browsing, they can sometimes lead to unexpected issues when excess data accumulates. Fortunately, we can resolve this with a few simple steps.

Let's start by signing into your QBO account using a private or incognito window. You can use the following keyboard shortcuts to save time:

• Safari: Command + Shift + N
• Mozilla Firefox: CTRL + Shift + P
• Google Chrome and Microsoft Edge: CTRL + Shift + N

Once you're signed in, create an inventory adjustment again. If it works without issue, return to your regular browser and clear its cache. Alternatively, you may also consider using a different supported browser.

You may also read this article to learn how QuickBooks tracks inventory: Understand inventory assets and cost of goods sold tracking

I've got your back if you have additional questions while doing an inventory adjustment, @acctggst. You can also ask any other QuickBooks concerns you may need assistance with. Simply leave a comment below, and I'll provide the necessary help.