Bug: if one scale dimension is negative, scaling that layer proportionally breaks your custom scale
Steps to reproduce:
1. Create a layer, of any kind (shape, solid, etc).
2. Twirl down to the layer's main Scale property, and toggle off Constrain Proportions (the chain-link icon).
3. Using the blue property text fields, type in a negative value for one of the scale dimensions (e.g., set X to -100%).
4. In the Composition panel / viewport, using the selection tool, hold Shift and drag the scale corners of your layer. Depending on the layer type, this will either make both scale dimensions negative, or make them both positive. This shouldn't happen. One scale dimension should stay negative, as you had it previously.