Fall of the Thran mana bonus reduction is not cumulative if cast multiple times on 1st chapter

Enygma6

My opponent (level 22 Teferi) in Training Grounds was using Fall of the Thran, and ended up casting the Saga 2 times in the same turn.  For each casting, it destroyed all other gems on the board.  But when I checked his mana bonuses during my subsequent turn, there was only a -2 reduction applied to each color from the values expected for the PW at that level.  
It appears that every time the anti-buff is applied, it is calculated from the base value for each of the color bonuses, not the present value.  
This is probably the same issue that Rishadan Port has, in that multiple triggers of the same color Activated Gem are not cumulative.

Tombstone

Interesting. I'll relay this to the team to confirm if this is intended.