Legacy Tigo optimizers are factory-configured in the "on" position, which means voltage as high as open circuit voltage (Voc) is present at the outputs as long as a PV module is connected*. In most cases, a PV module connected to a failed optimizer will produce power just as it would without an optimizer connected.
For TS4 units, whether F, S or O, due to rapid shutdown requirements, upon optimizer failure there will be no output voltage present.
Furthermore, the TS4 units as well as legacy optimizers, have built in string bypass ability. This means that whether an optimizer shuts down or not, the rest of the panels in the string can continue producing power as normal.
*For an MM-2ES Dual Maximizer with two PV modules connected, the output will be the combined voltage of the two modules in series. For Smart Modules with reduced Voc, check your datasheet.