This is related to #235: When I have the following document:

{
  "@context": {
    "@vocab"  : "https://vocab.getty.edu/",
    "a"     : "https://vocab.getty.edu/aaaaaaaaaat/"
  }, 
  "@id" : "https://vocab.getty.edu/aaaaaaaaaat/5001065997",
  "@type": "https://vocab.getty.edu/aaaaaaaaaat/datatype"
}

By point 3 of the spec, because https://vocab.getty.edu/aaaaaaaaaat/5001065997 contains the value of @vocab, it gets compacted as aaaaaaaaaat/5001065997 without even looking at the prefixes. I think this is not reasonable, in this case a:5001065997 would look much nicer IMO.