The player must be scouted to see their other positions. If he is natural in AMC it would only show that but you could discover the other positions by scouting. If he is already scouted then I do believe the problem is with his position going back to how it was before. I think it might have happened to me as well. Personally it's something I dislike because you spend a lot of time making them natural, you should be able to train them in another position and not have them lose what they learned. If this is the case then it would mean you could only train a player in 1 different position their whole career.
If you train them up to natural, stop training them, and continue to play them in that position, then you're fine. If they stop training and stop playing in a new position, then it will always decline.
Highly versatile players remember new positions better than less versatile players, so will decline more slowly.
I think the idea was to prevent players from accumulating lots of redundant positions - because there is a CA cost to excessive versatility that will see a player's attributes drop if they learn too many new positions.
Yes, it's one of those situations wherein it somehow makes sense that a player can be "natural" at something and then suddenly forget how to do it naturally. Nothing worse than spending a year or more training a player to become "natural" at a position and then have them revert back to "accomplished" the moment you stop their training...whereas a player with a natural rating in DB never seems to forget how to play their positions...