It's my pleasure to help check your sales tax calculation in QuickBooks Online (QBO).
If you're creating an invoice and manually overriding the sales tax amount, when selecting Tax or putting a check-mark on the Tax column, the amount won't be zeroed out. To single out the cause of this unexpected behavior, I suggest accessing your account using a private browser and start working from there. If it works fine using a private browser, I recommend clearing the cache of the browser you're using.
To start with, access your account using an incognito or private browser. You may use these shortcuts:
Google Chrome, press CTRL+ Shift + N
Mozilla Firefox: press CTRL+ Shift + P
Internet Explorer: press CTRL+ Shift + P
Safari: press Command + Shift + N
Also, using another supported browser can help us narrow down the cause of the issue.
On the other hand, if you're not working with invoices, can you please attach a screenshot on the page you're working where the box was zeroed out when selecting Tax.
Additional details would be much appreciated. Just add a post/comment below, I'll be right here to help you. Have a wonderful day ahead!