mocked method not called after zip when method called in service class

Question

I have service code like this:

forexample

1. Mono.zip(map(string, object), stringlist, customers).zipwhen(resp->
2. custom.readdevice("abcde", false).collectmap(sss->sss.getdeviceid()))

where custom.readdevice("abcde", false) is a mock method

I mocked this method in test class

Device device = new Device();

device.setdeviceid("DD");

Flux fluxdevice = Flux.just(device);

Mockito.lenient().when(custom.readdevice("abcde", false)).thenReturn(fluxdevice);

when I run this code until line1 code getting executed after that mocked method not called

if I try mocked method outside Mono.zip it executes very well but not in Mono.zip

please help me