It depends on the PV module's orientation to the shade event, that is happening at it's location.
PV Module Diodes
Most solar panels are comprised of silicon wafers arranged in 3 parallel circuits (or strings), with passive diodes that control the direction of current coming from the strings. The diodes act as one-way-doors, preventing current back-flow (caused by shade resistance). This causes the shaded string to remain uni-directional, until it can no longer contribute to the production of the panel.
For Example: A hard shade event over one string in the panel, can contribute to resistance or reverse current in the remaining strings. At this time, the shade-affected string's diode effectively shuts off it's section of the panel, so that there is no affect on the production coming from the remaining 2 strings.
Since these strings usually run vertically through the panel, It's important to calculate how local object or shading events will affect the panel. A PV Module's orientation can greatly contribute to diode reaction. If hard shading falls across multiple internal paralleled circuits, it may cause the PV Module to shut off more diodes than necessary.
The 3 images (below), depict how hard shading can affect the PV Module, when arranged in different orientations.
Diode Configuration - The 3 diodes control the vertical strings, internal to the panel.
Vertical Orientation - A horizontal shading event has shut down all 3 diodes.
Horizontal Orientation - A horizontal shading event has only affected one diode.
Which orientation is best?
Environmental conditions can vary from system to system. A PV Module may be better served in vertical or horizontal orientation, depending on how the shading object is casting it's shadow. If the falling shade blocks a majority of the area dedicated to any one of the diodes, that diode(s) will shut off. In cases where there is consistent shading across all 3, then the entire PV Module may fail to produce any voltage.
Other design considerations may contribute to the success of your system. To learn more:
Designing your TS4 system for different tilts, angles, and orientation
How does diode activity affect Tigo Product?
The Tigo MLPE (optimizer) requires 10-12 volts minimum, in order for it to remain active. The MLPE units source their running voltage from the PV Modules. So, If the PV Module produces less than 12 V per Diode, then the MLPE unit attached to the module may shut down (due to low Vin), even though the shade is not covering the entire module or diode section.
If a low Vin condition MLPE shut down occurs, the system will cease to provide data for that unit. This will be shown on the Tigo Layout, as grey module (no communication/no data).
For more information: PV-Module Colors - What do they mean?