Boiler dont heat to maximum temperature when low energy density fuel like bamboo is used.
1)Bamboo


2)Dried kelp


3)Coke


Reproduction

1. Build advanced high presure boiler like on image
   
2. Supply water and bamboo (AE chest and export bus used for bamboo suply only)
   
3. Look on temperature of boiler
   
   

This a feature, because the boiler can only burn one item per tick and because 1 bamboo per tick is nout enough to reach its maximum output, the temperature doesn't therefore reach its maximum (the output is proportional to the temperature).