Thats intended. On our server it works similar to official server, because flametongue weapon +(40% damage) and searing flames (+100% damage) stacks additive, not multiplicative. I test it 1 year ago on official server. How it works:
For example, my damage from Lava Lash is 10000.
With Flametongue weapon, will be 14000. (10000*1.4=14000)
But with flametongue+searing flames, it will be: (10000*1.4)+(10000)= 24000
Declined.