I edited before seeing your replies.
I missed that death rattles do trigger on Saps to full hands.
I bet Stalaag & Fuegen have special rules outside the normal Deathrattle code. Since you can silence the first and the second still triggers the summon their condition isn't based on the death rattle. I bet when a minion dies to sap+full hand it's a different process than normal death or normal card burn. Legit bug.
I wonder if Bolvar Fordragon has the same bug. When in hand does his attack increase if you sap+full hand a minion from the board.
Also apparently Ben Brode doesn't know deathrattle works as well as you, so let's not kiss his boots just yet.