There are a lot of characters that obviously skim off quest rewards, but there's no effective way to call them out on it because the rewards are random, and there's no way to know what to expect.
Therefore, I would suggest that instead of the current "random amount assigned at end" could it me modified to be a random amount to be announced by the questgiver upon taking it, or to have some other mechanic to give an idea of how much gold is being given out.
Stealing part of quest reward should happen only with DM supervision, IMO.
Theft is a hostile act that can initiate pvp, and in front of an NPC, a DM should be present.
More, the thief is exploiting the fact that party members can't ask the NPC "how much gold did you give us?" if no DM is present.
If you let someone run ahead and grab the reward, an divide it far from the NPC, that's another issue.
My 2c.