Uncollide-able parts do not drag like they are supposed to.
When you select a model, highlight a part inside the model while the whole model is selected; the selected parts inside the model tend to offset upon dragging.
Here’s a clip showing both processes at the same time:
The problem can be reproduced by grouping a model with uncollidable parts. When you drag the model these parts go straight through others. When an entire model cannot collide the whole thing goes straight through other bricks.