I checked all my previous invoices it turned out that things were ok up until July 2022, then the problem occurred and carried on since then in every single bill.
I am still waiting for the customer service representative to get back to me on this. Meanwhile, if you guys have two rates meter kindly check your invoice and report here if you have the same issue.
This is the second time I discovered an issue with their billing system. The first time was the difference in tariff between the invoice and the account which I explained here:
Economy10 Meter Confusion
And yet no following up from them and no solution.
I believe E.ON made good money from people who never check their bills, it is always easy to blame the energy prices and the war!