i've read about this being an issue - they are a short coarse thread. I think mentioned in an article by John Healy on 'vintage bikes' re Triumph cranks. at that suggestion I put some thread sealer (i had some loctite 567 nearby) around the inner section of the female thread. then some threadlocker near the outer of the plug thread (tho John says not to) and punched the face of the plug (not the crank) for good measure. however I did not air test it for leaks - reading your post I wish i had - i just stuck a piece of wire up from the bush side to ensure it was not blocked.
FYI John had written to use a "little bit of Hermatite Blue sealant, ThreeBond #4, or Loctite 515 or 518 anaerobic gasket sealant" on the thread.
Loctite 567 is a slow curing coarse thread sealant so sounded ideal to me.