Seemingly buggy behavior with PySCXML: the second event "f" is not received in most executions

gistfile1.py

import time

from scxml.pyscxml import StateMachine
import logging
logging.basicConfig(level=logging.NOTSET) # show detailed debug info

xml = '''
<scxml xmlns="http://www.w3.org/2005/07/scxml" version="1.0" datamodel="python" initial="S1">
    <state id="S1">
        <onentry>
           <log expr="'hello S1'"/>
        </onentry>
        <transition event="s2" target="S2">
           <log expr="'transition s2 from S1 to S2'" />
        </transition>
        <onexit>
           <log expr="'bye S1'"/>
        </onexit>
    </state>
    <state id="S2">
        <onentry>
           <log expr="'hello S2'"/>
        </onentry>
        <transition event="f" target="F">
           <log expr="'transition f from S2 to F'" />
        </transition>
        <onexit>
           <log expr="'bye S2'"/>
        </onexit>
    </state>
    <final id="F">
        <onentry>
           <log expr="'hello F'"/>
        </onentry>
    </final>
</scxml>
'''

sm = StateMachine(xml)
sm.start_threaded()
sm.send("s2")
#time.sleep(1) #problem does not occur when sleeping in between
sm.send("f") #event is ignored in most executions, statemachine stays in state S2