It could be cracked at the threads (seen this before too.)
Look very closely between then sleeve and the threads and see if there isn't a crack or stress fracture (vertically.)
Just for extra insurance, I'd throw a stock graphite (soft) Briggs gasket on it as well -- they conform to some deck irregularities better than the stiffer (Cometic mls, FelPro, metal-clad, etc) gaskets do.
If you've got a good flat & smooth RA finish on both the block and head, then I prefer the Cometic or Fel-Pro gaskets.
Glad it's runnin' good for you either way.