Platform, device version and operating system: Steam, Windows 10, GoW version 4.5.0.07
What you were expecting to happen, and what actually happened: Undine’s ability “Ocean’s Ruler” is boosted by Merfolk and Submerged allies. Undine’s trait Tidal King submerges him at the start of every turn. Therefore, he is submerged before you cast his spell. Because of that, when you look at the Boost Ratio, it counts 8 damage boost from him being a Merfolk and another 8 for him being Submerged. So it reports that he’ll do (Magic+5)+(Submerged Allies8)+(Merfolk Allies8).
In the screenshot above, I have 3 Merfolks (Leviathan, my Hero, and Undine – Aquaticus is somehow just an Elemental/Mystic, not a Merfolk), and one Submerged troop. It says his Boost is +32, on top of the 29 damage. That’s 61 damage, but if you cast it, he’ll only do 53 damage, because when he casts, he loses his submerge and it isn’t included in the boost anymore.
In other words, Undine’s reported Boost damage is always 8 higher than it actually is if he’s Submerged.
How often does this happen? When did it begin happening?
All the time, but more specifically it’s only noticeable because of the Boost Ratio reporting how much damage it does.
Steps to make it happen again
Allow Undine to submerge himself (or submerge him from another troop, like Aquaticus’ trait), then go to cast his ability, the Boost Ratio will include his Submerge status, but when you cast it, it will be 8 less damage than reported.