I too get this complaint often. I’ve never figured it out. Customers say ‘I’ve baked the same cake for years and not changed my recipe’ . Must be a change in the oven but I’ve never got to the bottom if it?
Used to get this now and then at Zanussi years back. The answer was to space the back panel ( on the side of the lesser cook) on one corner to allow better air flow ( I used to use something like a 7mm nut around the screw).
if everything is working correct, the only thing it can be is air flow, even allowing for a fan oven rarely being exactly equal in cooking.