I believe the furin cleavage site has been quoted to be an exceptionally rare mutation. It is extremely unlikely for this particular mutation to exist without being accompanied by other less-rare mutations in a natural-spread scenario. For this particular mutation, a gap of greater than 3.8% would be expected. It may (I'm not an expert) may be unlikely for a 3.8% drift, but is less likely for only a 3.8% drift given the rarity of the ACE2 spike.