What you were expecting to happen, and what actually happened:
17 Damage should of been done to the AI, but only 14 damage was done.
How often does this happen? When did it begin happening?
It’s been happening for four months at least (probably been this way since release or since the explosion Mana reduction). It’s just inconsistent so it took me a while to show proof.
Steps to make it happen again
It’s inconsistent, so it has to be cast several times. The boost ratio will work correctly sometimes and not others. Which is why I feel like the 50% Mana from explosions may play a part in the bug.
From other spells that have been similarly broken in the past, the first thing I’d check is if the boost was accidentally associated with Red or Green gems. The second thing I’d check is if the boost ratio was incorrectly set for the text and is actually x2 (the text seen here is at least one abstraction away from what is actually set for the spell in the database and can have a display override, though it would be odd for something intended to be a x2 to have a special case display override.)
Explode > get boost ratio is such a base level thing that we should be seeing this issue everywhere if it were just ignoring the center gem. Unless every different spell that explodes into getting boost ratio uses a different implementation, in which case, yikes…
Lava Elemental and Obsidian Golem are two troops that have very similar spells, and should work exactly the same (as Possessed Urska from above). Alas, they both include the centre gem in the calculation for boosted damage: