Masked layers in the gesture playable layer will always control the masked parts if any animation is active in the entire gesture playable layer. For instance, if 'desktop arm control' has been set up in a layer masked for an arm but is not currently in use, and a gesture is used that activates a hand pose in the "Left Hand" or "Right Hand" layers, the arm will motorcycle pose. This is the case even though the layer playing an animation is only masked for a hand, and the 'arm control' layer is only masked for the arm.
The issue is not apparent with the default left and right hand layers because the hands are always animated. However, features like 'desktop arm control' are only active when in use, and you would otherwise want the locomotion animations to take back control of the affected parts.
The issue is compounded further by the well-known unity bug related to empty states (no animation clip) when using write defaults off. Since an empty state is not possible without causing other issues, if you try to use the common workaround of a 'placeholder' animation that animates a fake property, this too will cause the bug because an animation clip is active. With write defaults on, an empty state can be used which does not trigger the bug. However the arm would still of course motorcycle pose when an animation clip becomes active in any other layer.
Setting up layers for things like 'desktop arm control' can be done successfully in the locomotion playable layer with additional masked layers when using write defaults on. This workaround is possible only because of the ability to use empty states, allowing the base locomotion layer to take back control when lower layers (e.g. arm control layers) are only writing defaults.