Killing the magic eaters appearing didn't give XP.

Is this intentional? Shouldn't be at least 1 XP or such awarded?

As with all enemies, experience is based on the challenge rating of the creatures you fight, the number of people in your party when you fight them, and other variables. Some people will receive XP for killing magic eaters, some will not. More powerful magic eaters are more likely to give you XP.

Actually, those specific creatures have their XP drastically reduced. Generally, in situations where you can summon/call/attract unlimited numbers of enemies, each individual enemy has their XP reduced to a negligible level in order to prevent farming. This is the case with magic eaters and also with certain creatures in a few other areas (best discovered in-game).