I’ve written extensively about the issue here, even addressing them directly, however, there hasn’t been any explanation yet. However, the data from the wiki [1] suggests that the reward in both cases is a random golden (legendary) portrait.
It’s up for discussion. I’ve explained my preference for a portrait, since they are much harder for me to come by as a F2P-player. However, good communication, apparently, is not a priority for them, which is very unfortunate.
[1] https://hearthstone.fandom.com/wiki/Mercenaries/Valeera_Sanguinar#Tasks