To state the obvious: It must be something wrong in the Emlid VTG data. Either the output or the readings. Or in the RS5 VTG reading.
Is it possible to connect another gps (another brand) to the rs5 to see if you have the same issue? If so, the issue is how the RS5 reads/interprets the VTG data. Another suggestion: If it is possible to connect the Emlid to a PC/PDA/phone and read VTG data, try walking north at 1 m/s and check if VTG data agrees. (Or: Return GPS and rs5 to manufacturer/dealer and tell them that something is wrong)