Port Aggregation & MTU

We have s6220 series switch. in iperf tests we have some fragmentations. so digged into deep and found this;
aggregated port mtu value is 9216. but the ports under it still seen as mtu 1500. and we cannot change this mtu value.
in s6500 series, ports mtu under aggregations also set as 9216 and there is no fragmentation.
so we are thinking this is the cause of the problem. how can we change ports mtu values under aggregation?

Thanks.

Dear sir，

Please share us your s6220 version and detail model. We will feedback this issue to our R&D