What version of ThingWorx are you in?
Also, please attach an export of your mashup entity for quick testing.
In Grid Configure Grid Column,
1. UnCheck AutoWidth for all Property.
2.Divide Grid width by number of Property. For example: If you have 6 property to display in Grid, divide Grid width 630/6 = 105, and in Configure grid column, give width of each property as 105.
I have the same issue in 7.2 (with standard styles) :
Did you find a solution or is this a known bug? I'm on the same version and have a grid in a responsive container so Velkumar's response doesn't apply.