Hi, well you seem to have found the answer, but for the record a Doughnut is Not a Seal, it’s designed to stop the cistern coupling bolts sitting directly on the pan, nothing to do with stopping water, it couldn’t it’s made of porous compressible foam.
Also anyone reading this, silicone sealant doesn’t help, I just had to replace a cistern and spent forty minutes, even using silicone dissolver to remove the old sealant before I could put the suite back together with a new Fluidmaster doughnut, so please don’t do this, silicone has its uses, this isn’t one of them!