Fragmentation support is limited to 50pps on EX4400 platform.
This KB describes the behavior.
For example, when the input packets are over 50 pps like 1000 pps on EX4400, the output packets are 100 pps.
Traffic is 2000bytes/1000pps------------------------------------------>
------->ge-0/0/10(MTU9000)[EX4400](MTU1500)ge-0/0/12-------->
> monitor interface traffic Interface Link Input packets (pps) Output packets (pps) ge-0/0/12 Up (0) XXXX (100)
Fragmentation support on EX4400 is limited. Anything beyond 50 pps is not supported.