I'm not entirely sure if all the behavior is a bug or on purpose, but I still think it's strange how it seems to work. I tested all of these using workshop as the gainer.
If I gain something to the discard pile normally and have multiple sleighs in hand, it only lets me react with one sleigh, no matter which option I choose.
If I gain a sleigh to the discard pile normally and have 1+ sleighs in my hand, if I pick the topdeck option, it only lets me react with one sleigh.
If I gain a sleigh to the discard pile normally and have 1+ sleighs in my hand, if I pick the put in hand option, it lets me react repeatedly with sleigh as many times as I want. I assume this somehow has to do with the fact that the composition of my hand is changing each time and that a "new" sleigh is being placed in my hand in each step. I still think it's weird that this happens, given the other behaviors.