Modify basic uas scenario with

<recv request="INVITE" next="200ok" test="var">
</recv>
<Reference variables="var" />

and put

<label id="200ok"/>

between 183 and 200.
UAS: sipp -i 127.0.0.1 -sf modified_uas.xml -m 1
UAC: sipp 127.0.0.1:5060 -sn uac -i 127.0.0.1 -m 1

Sipp should continue the scenario flow.
Sipp 3.4.1 also has this problem, works correctly with 3.2

This was useful for testing call queues when you feed var from csv file.