Roads / Walls Library Research effects bug as age progresses

[Jackuriki]

as my age advances the effects of wall and road researches has gone... example before i reach gunpowder my wall was 95/90.. then when i reach gunpowder it says it adds +10 wall among others but when i build it it only gives 5 and becomes 95/95 max limit... it should be 105/95 because i have already researched 2x +5 walls in the library.. i will post screen shots when i get home later