Air appear when piston push kelp in water

At that time, I was working on a fully automatic kelp machine. When the piston pushed the kelp, air appeared, which affected the growth of the kelp ( and appeared many times )(I don't sure if the too high random tick speed make that)
Expected Results:Normal growth of kelp
Observed Results:Kelp cannot grow due to air caused by piston
 
https://v.afbcs.cn/XhC32J
this is the attachmen