My son also had the same problem for the past 2 months. When my mom said "goodbye" to him, we would start to :-( :-( but he can stop shortly after my mom really left.
I asked the teachers regarding the condition of my son in school and they also replied that he is good, happy and active.
However, there is a change since this week because he can say "goodbye" to my mom happily without :-( :-(
I think you are no need to worry because the adpation of children are different.