"Speed over ground" error in GPRMC when use 5Hz u-blox M8N output

[ghost]

Hi

In PPP and Single mode, I found if u-blox M8N runing at 5Hz, "Speed over ground" filed of $GPRMC output will wrong, like this:
$GPRMC,075725.80,A,2539.2619141,N,12406.5313944,E,919.95,31.24,130716,0.0,E,A*09

but if u-blox runing at 1Hz, $GPRMC output is correct.

my RTKLIB version is 2.4.2p11

Thanks by advance