Rest assured that seeing the System Administration user in your QuickBooks Online (QBO)'s Audit Log page isn't a glitch in the system, @anna54.

Yes, you're right. Invoices are always tied to a user. However, when you modify an entry that also affects another entry the program will treat the activity as an automatic system update. That's why you're seeing that the action is made or added by the System Administration on the Audit Log page.

An example of this scenario is when you edit a payment to attach to a different invoice. The action will automatically be recognized as a system admin update. The System Administration may show up on the said invoice saying payment is applied or an “Indirect Edit” by the user who initiated the change.

Additionally, you can refer to this article to learn more about what the audit log can do and how to use it to keep an audit trail: Use the audit log in QuickBooks Online. It includes a detailed explanation about other special users such as online banking and import administration and support representatives.

Please know that you're always welcome to comment below if you have other concerns or follow-up inquiries about monitoring your transactions in QBO. I'm just around to help. Take care always.