I was more annoyed by the 'successful' lore check telling the player that Pixies were sometimes malicious, so you never know... when that is categorically not true, and it's an intrinsic and fundamental part of the nature of pixies that they abhor violence and confrontation, and are good-hearted creatures - mischievous at times, but by definition never malicious.
I've been wondering about that part, too. Like you said, 5E Pixies can be mischievous, but they are not described as being malicious.
If they wanted to use a creature that might be helpful or might not, and create that sense of risk, then they should not have used a Pixie for it.
And if a Pixie specifically is needed for this lantern to work, but they wanted to create a sense of risk for the player, maybe they could have added an insight check, that the pixie seems to be quite angry (understandably so,
I would be angry if I were imprisoned in a lantern).
