I don't think this is a bug as much as it is a programming error on the part of PD.
When setting a weight restriction in a lobby, the setting enforces it as a maximum allowed weight as opposed to a minimum allowed weight. This is backwards from most, if not all, racing series in the world. Most racing series have rules that contain a minimum weight.
Example of how the setting in GTS works: If I set the weight limit to 3000 lbs., I can take a car that weight 2900 lbs. and enter the race. If I take a car that weights 3100 lbs., I am denied entry based on the weight of my car.
This is backwards from most racing series and it's backwards from the setting that was in GT5 and GT6. If PD has a good reason for changing this then I'd love to hear it. If, as I suspect, it's a simple programming error, it should be a simple fix that can be taken care of in the next patch.