QBO changed something with sales tax recently in Q1 2025 and really mucked up things.  It appears now to get sales tax to charge correctly you have to mark the item as taxable at the item level, and then mark it taxable in the invoice as well.

Of course, I have many items I need to update now and their API isn't working which I wrote a program to bulk update my items with.  It worked when I updated price, but it doesn't work with the taxable flag.

That's what you get when you move away from professionals like those who did the PC version of QB and go to the cloud I guess...