I'm pretty sure you need to have actual space in your inventory to receive the runestone because they are not designed to go into your loot list. If they did go into the loot list, it would be possible to give it to party members which the system of special faction levequests chest rewards does not wish to allow. I'm going to guess it got lost when attempting to go into your inventory just as it hit full. I hope you get compensated for it though because I would be very angry about losing one of those seeing from experience how rare they are. I'm pretty sure the name of the guildleve was what was asked for which is "Wanted: Mammoth and Master" I am assuming.