(I'm obviously not an electrical engineer, given this is 10x stuff) If we're talking a 2.45 GHz microwave signal, that's a 12.2 cm wavelength.
But I thought for shielding you only needed to have gaps of <wavelength to null emissions.
Is there some fractional-wavelength propagation, or is my understanding of EM shielding off-base? How are microwaves noisy? E.g. https://physics.stackexchange.com/questions/269672/does-a-fa...
It is "both* dimensions that must be a long way below the wavelength to keep microwaves in.
Instead the door seal uses a technique called a quarter wave choke. It relies on reflecting back any microwaves wanting to escape, and by making incoming and reflected waves perfectly cancel, no power is transmitted.
As OP said, even a small amount of soup dripped onto the seal and your microwaves will all start escaping.
"Propagation" is probably not the right word, but fractional wavelengths can "leak" some amount of field.
Unfortunately there were a few microwaves sold recently in Europe which forgot that design element, and if you pointed at food while cooking it, you would cook your fingers too.