Bad service, Warranty claim rejected if quality plumber work on geyser

To whom it may concern

I logged a claim at Franke on website for warranty geyser on 24/11/2018 13h06 Geyser serial number: 550 ********** 72, manufacture date 10/05/2016, whereby they rep**** that someone will get in contact with me as Qualified plumber. Client was without geyser for 2 days and we replaced it with Kwikot geyser

Now on 05/12/2018 I called Franke myself to follow up on claim just to hear if a qualified plumber worked on the geyser the warranty is forfeited

Please explain to me the following, why can a qualified plumber replace element after 2 years when it burned thru and warranty is still in tact but if you replace it the warranty is forfeited???????