The game is doing a semi-turn based combat coming to the player doing either weapon or magic attack, and the enemy doing a melee attack.
But when the enemy can cast spell (ranged attack), or there's a trap involved, those will be cast real time, hitting you 2-3 times while you are casting your spell (melee animation is faster, there the problem is less noticable).

As far as I know, the game features a real time combat system: however, during combat, animations may be shown (particularly when a spell is cast) and when these animations occur, the time flow is temporarily suspended, meaning that other actions are not allowed. Different spells may have different casting time, though.

Anyway, I don't think this could be considered a bug at all.

If it would be normal behaviour, then melee attacks wouldn't stop either during player-spellcasting. But they do.
Thus definitely a bug (or engine-limitation, which is still a bug).

Not at all in my opinion.
A bug makes the program to behave in unintended ways, while this could be a deliberate choice by the developers to favor magic attacks over melee ones.

Except they do not favour.
For the player both work the same way.