Courtesy of another player posting similar issue, and a response from someone else 
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I’ve managed to narrow this down. If you use “T” shortcut to view terminal overlay, the buttons are fine they open the little window below the terminal summary window to assign vehicles or staff.
If you select the terminal overlay icon on the bottom menu, and then click icons shown in my original screenshot, they do not show.
Pleased to have found this out so thank you to those replying to Unable to open the terminal "staff/vehicle assign" windows - #5 by craigwats