Odd. You probably need a pro to take a look. Could be simply main voltage sagging from the high draw and those LEDs are especially sensitive.

Is the oven neutral touching the lighting circuit's ground (they look close, can't tell if they're actually in contact)? If so, my wild theory is that it might be transferring enough current to cause thermal expansion under the lug where the lighting neutral and ground are double-tapped (very common but not a good idea BTW). That could theoretically loosen the lighting neutral's connection after a time and dim the lights. Normally the oven neutral wouldn't transfer enough current to matter, but aluminum combined with a loose connection could make the copper path more attractive.

But whoever gets in there will probably find a more boring explanation. Good luck.