I'm here to help you sort things out, corina-felber.
So far, we haven't received any similar issues reported. QuickBooks Online offers automation features, such as automatically applying payments to an invoice.
If this feature is turned on and your customer has an available credit, this will automatically link to your open invoice and marking them as paid.
To turn off this feature, here's how:
Click the Gear icon at the upper right corner and choose Account and Settings.
Go to the Advanced tab in the left panel.
Select Automation and remove the check-mark for Automatically apply credits.
Tap Save.
Hit Done.
If you want to unlink the payment, see the steps below to complete the process.
Locate and open the invoice.
Click the 1 payments at the upper right corner and select the linked transaction.