We recently identified an issue causing certain towers (Fire Flak, Dark Flak, and Fire Turret) to not increase in attack power after around level 40 (they still gained health, but did no additional damage). We released a fix for this issue in 3.90, which we failed to properly communicate in our patch notes. We apologize for this. After releasing this fix it became clear that the >40 towers were not balanced, so we are rolling back the fix while we focus on rebalancing the towers (and possibly dragons).
What this will mean for players is that you will not see any change in attack stats for the above towers if you level them past around level 40, until we get things rebalanced.
UPDATE: This update has been completed, and the towers should be where they were before 3.90