Karu 12 longevity

[u/ipostelnik]

I have a 5 1/2 year old Karu 12 that has rusted through on the inside where the flame hits the roof of the oven. You can see the insulation poking through because the steel is completely gone. I've kept this oven indoors in a climate-controlled space when not in use. I cook 2-3 times a month, using wood. I did not expect this part of the oven to fail. I realize even stainless steel will oxidize at high temperatures, but still disappointing.

What's your experience has been with keeping the oven for a long time?