Dear Kumura,
The FAST-S-Function will use extrapolated values of the inputs (including blade-pitch angle) passed from Simulink (as discussed in other forum topics e.g. S-Function input output signal mismatch - #2 by Jason.Jonkman). Thus, the FAST S-Function may not report exactly the same values of the blade pitch angles that are input from Simulink. I would always ensure that (1) the initial pitch angle set within the ElastoDyn primary input file matches the pitch specified within the Simulink model at time zero and (2) the pitch angle changes are defined smoothly within Simulink.
Best regards,