Canon 5d4 Focussing Issue with TTL Flash Help Sought

[Eddie1]

I'm having trouble with my ( usually reliable ) Canon 5d4.
At 160th sec / F5.6 with 24-70mm lens set to about 50mm and using off camera flash ( profoto b2 ) set to TTL almost half my images are out of focus / with slight motion blur.
And I have no idea what's causing this.
At 160th sec it's not camera shake it seems to be something to do with the flash sync.
Anyone any thoughts or had a similar issue please let me know how you resolved the problem.
Thanks.
All input Much appreciated .

 

[WD40]

If it is something to do with sync, why are half the photos affected but the others not? Have you tried the same settings and environment with another body?

One shot or Servo AF?

What is the subject? What ISO?

Does the exposure look any different between good / bad shots?

 

[Phil V]

does the transmitter have focus assist? are you using continuous or one shot? hows your usual technique in these conditions. what ISO? if the ambient is too high you'll risk motion blur.