It is a result of the flexibility of the event mechanism.
Wofost72_PP runs a simulation for the potential production level, as such there is no handler that will react to irrigation events.
However, that does not impede you from sending irrigation events. But given that there is no handler, there is no code that will actually look at the contents of the event so nothing happens.
Only in case of the Wofost72_WLP_FD there is an event handler that looks at the irrigation event content, so the contents must be specified as a float.
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