Was this a glitch?

[Mirage]

If you have earth elemental junctions and the damage reduction adds up to exactly 100%, i believe it is possible that the game simply shows nothing instead of a "miss" or "0" damage.

You might be right, but this definitely didn't happen with me, because I didn't have hardly any earth magic, plus it happened to all 3 of my characters, plus in a separate battle later on (with the same stuff junctioned) the move did take damage. So it can't be to do with what I had junctioned.

why do you think this is like cheating? it's not your responsibility to iron out coding errors.

True that it isn't my responsibility, or fault. But I view it like cheating because the end result is that I defeated the creature, and all creatures after it until I healed, with more health than I should have had.

You played the game according to the rules set down by the programmers. Not cheating. Exploiting bugs isn't really cheating either, it's just taking advantage of rules that aren't defined well enough.