New Game with most of the starting party.
Cassandra - Sorcerer Level four only has 6 Spell Points
Gene Eric - Cleric Level four has 28 Spell Points

Is Cassandra bugged, or was I unlucky on the level ups ?

It's likely that Cassandra had low intelligence when she leveled up. Try resting and see if it fixes the issue.

I note that maximum SP doesn't really matter in this game, as it is re-calculated every time you rest, and other effects that give you SP ignore your maximum.

I've run into the same issue before, not sure what causes it. Also, dtgreene is right, resting should resolve the problem and reset all party members spell points to their correct value based on their level and statistics.

Also check if Cassandra is afflicted by a status effect. There's one (disease, I think?) that will prevent the restoration of spell points, and may even lower the maximum until cured. Visiting a temple and paying them can resolve that for you.