The one-event delay is usually caused by the absence of a Single Server block with zero service time that causes operations to be ordered correctly. Insert this block after the Get Attribute block that is feeding attributes to Stateflow chart, but before the block that is sending the function-call to the Stateflow chart. This will ensure that by the time the function-call reaches Stateflow, the attribute values are available.
Note that there is some configuration you have to do to your Stateflow chart in order for it to behave in event-driven fashion with SimEvents. More on that is here:Using Stateflow Charts in SimEvents Models
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