What version of ThingWorx are you in?
Also, please attach an export of your mashup entity for quick testing.
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.
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) :