The system added the VAT rate of the invoice to the expense receipt - why?

This is because an invoice can only have one VAT rate. If you have checked the item "invoice from the customer" on the expense reimbursement form, the expense receipt is added directly to the invoice as a line.

If you invoice expenses from the customer, the VAT rate of the expenses is also determined according to the VAT rate of the work you do.

If you don't want this to happen, you can either include the portion of the expenses directly in your invoice amount or make an extra line on the invoice manually. In this case, you can also edit the amount of the expense to be correct, but please note that value added tax will be deducted from this amount according to the correct VAT rate.