I also tried to reproduce this bug and could not do it, it seemed to work as expeced:
Turn 12 - Ingix
I plays a Herald
I draws an Estate
I reveals a Doctor
I plays a Doctor
I names Estate.
I reveals a Copper and a Herald
I topdecks a Copper and a Herald
(remark: only reveals 2 cards because deck is very small)
The problem may be that during the choice of which card to select for Doctor all cards in your hand have a green border (are legal to be selected). This is very similar to what it looks like when you can play an action (your playable actions have a green border). Since Herald is a "surprise" card, I think it possible that the original poster saw the green border around his Herald when the Heralded Doctor resolved and clicked on it, because he wanted to play it. Instead this was interpreted by the game to mean "Select Herald for Doctor".
Turn 12 - Ingix
I plays a Herald
I draws an Estate
I reveals a Doctor
I plays a Doctor
I names Estate.
I reveals a Copper and a Herald
I topdecks a Copper and a Herald
(remark: only reveals 2 cards because deck is very small)
The problem may be that during the choice of which card to select for Doctor all cards in your hand have a green border (are legal to be selected). This is very similar to what it looks like when you can play an action (your playable actions have a green border). Since Herald is a "surprise" card, I think it possible that the original poster saw the green border around his Herald when the Heralded Doctor resolved and clicked on it, because he wanted to play it. Instead this was interpreted by the game to mean "Select Herald for Doctor".