Does anyone want to tackle this behavior? I would call it a bug, but whatever.
This occured in both the SNES and GBA ports..
Here's a quick background of documented behavior before I get to the bug.
When a Behemoth is alone, and a Holy elemental spell is used, it counters with Maelstorm/Storm.
If Holy is reflected, obviously the games behaves differently between the ports..
SNES - Behemoth counters with physical attack
GBA - Behemoth counters with Maelstorm
If a summon is used on two Behemoths... it also behaves differently..
SNES - One of the Behemoth decides to counter
GBA - Both Behemoths counter
Kinda new info...
If Meteor is cast and kills 1 of 2 Behemoths in both versions, the Behemoth that is alive counters with Maelstorm.
If Meteor is cast and kills a Red Dragon in a Red Dragon+Behemoth group..
SNES - The Behemoth counters with a physical attack
GBA - The Behemoth counters with Maelstorm
Onto the bug/behavior...
When a Behemoth is not alone, and Holy is reflected onto the Behemoth.. the monster does nothing.
My question is

It occurs in both SNES and GBA ports.
Note: When a different spell is reflected onto the Behemoth, such as Bio/Virus, it counters with a Physical attack.