In September/October of 2019 all cards that previously had "changed their appearance" when played to somethign else, got errata to no longer do that and instead play another card in a special place and leave it there. Look at http://forum.dominionstrategy.com/index.php?topic=19893.0
for the overall picture.
Among them is Inheritance, which previously changed Estates to become mostly another card, but which now reads:
Inheritance: Event, $7
Once per game: Set aside a non-Command Action card from the Supply costing up to $4. Move your Estate token to it. (During your turns, Estates are also Actions with "Play the card with your Estate token, leaving it there.")
So now the only change is that (on your turn once you bought Inheritance) the Estates are also Actions and have the ability to play the card with your Estate token.
That means, in your case. You first played a Coven, then the Estate as second, which in turn played the Conspirator with your Estate token, which means it is the third card you played, so you rightfully got the bonus effect for Conspirator.