Try with property scope as "default/synapse"
FiveO edit comment:
Try with property scope as "default":
Sending a transport property from the inSequence to the outSequence (on behalf of the default scope):
<inSequence>
...
<property name="myPropertyInTransport" value="myValue" scope="transport"/>
<property name="myPropertyInDefault" expression="get-property('transport','myPropertyInTransport')" scope="default"/>
...
</inSequence>
<outSequence>
...
<property name="myPropertyInTransport" expression="get-property('default', 'myPropertyInDefault')" scope="transport"/>
<!-- Now myProperty is also available in the outSequence -->
...
</outSequence>