From what I've read it popped its head up in early trials of some vaccines on animals. Those vaccine paths were quickly terminated and have nothing to do with current approved vaccines, at least AZ or Pf.
So far to date, there's not evidence of ADE at all in current vaccinations, and yes studies have been conducted as it is a serious question. However there is a slim possibility of say next years strain being different, and that may or may not be an issue depending on who you ask.