I have similar questions which I expect to be blown off by those in the business. If the winner refused to pay for the coin (or was unable) or if he had bid under some sort of purchase limit through which his winning bid was canceled, I would expect not only his winning bid to be erased but all of his lower bids that pushed you up to the level of your last bid. If the two of you were the only bidders on the lot, you might have won it at start but allowing his fake bids considerably increased the realization. Of course erasing those bids would do wrong to the consignor since someone else might have bid against you had they not seen the unbeatable winner was bidding. It would seem a "can't win" situation. I would have in the very least thought it appropriate for you to be contacted by the house and had the situation explained offering you options rather than just adding the coin to your tab. Problems like this are part of any business dealing and how they are handled show us who are the professionals and who are not worthy of long term patronage.
I am thinking it was more of a computer glitch. During an auction you are sitting on pins and needles/ esp. when your bid is leading. On the Roma site/ prebidding was at 2600/ I bid 2800. so a green box shows you winning/ again I was outbid with red box showing that @3000/ I bid 3200/ green winning bid came up/ then the red outbid came on screen/ so I gave up. But thinking back now I realized "bid again " came up @3400. Now, I realize, it should have been 3600/ since 3400 would have beaten my prev. bid of 3200 Sounds complicated, but I am sure that is what happened. In the end, I was very happy with my Roma results.
I have never heard of a case where the winning bidder did not pay so the auction house later declared the underbidder the winner without approval. As you suggest there are a million reasons why that would be bad. I have seen cases where whoever is running the online software accidentally records a room bid but then retracts it back because it was an error or misunderstanding. Can also happen when a room bid equals or is very close the book bid, and it is at the auctioneer's discretion what to do. I suspect a "glitch" like that is much more likely.
I think their bidding platform is manually operated, so it could have been a misclick from the clerk or staffer operating the terminal. I saw it happen a couple of times through the auction, and you could hear instructions to update the website bidding over the audio