I have been searching for a solution to this problem for a while now and I’ve just figured it out! In the material of your widget component, set the ‘Translucency Pass’ in the details panel of the material to ‘After Motion Blur’ - this will solve the issue.
Can confirm, this does remove motion blur from that material. If you can’t change the Translucency Pass option, just set the Blend Mode of the material to Translucent.
Another option I stumbled across that worked for me was to set the Output Velocity in the material translucency options to enabled. I assume this only works when the Blend Mode is already translucent. I am working with the engine (UE 4.27.2) provided Widget3DPassThrough material instance.