Please take a look at the attached screenshots. The simulation data for all human tasks are:

Distribution Type:     Uniform
Processing time (max): 8
Processing time (min): 8
Staff availability:    2
Working hours:         8

The expectation is that the first task has no wait time, if the simulation is started with 8 hours interval. This is true for a process where the tasks are handled in sequence, and even for a process with one level fork/join, but not for the attached process with two levels.