Traffic over 3.2-3.3Gb not show

Recently upgrade uplinks over 3Gbit and notice strange thing - when traffic exceed 3.2Gb dude start to show wrong speed(near 100-300Mbit).
snmp data is right(checked oid’s for in and out octets), third party snmp monitor show speed normally.
meybe some overflow in rate function?

dude server at
RB1100x4AH dude edition
RouterOS v6.40.5

monitored device - cisco 6509

Good afternoon!!!
Сonfirm the problem.
x86_64 CHR 6.40.5 (stable), Use SNMP ver2.
Qtech QSW-6500
traff.jpg