cap2hccapx evaluates the beacon and the proberesponse to get the essid and essid_len. That's not enough:
On hidden ssid's, mailformed packets, and/or packet loss of proberesponse cap2hccapx fails to get the necessary essid.
It's a better way to get essid and essid_len from reassociationrequest or associationrequest which are sent immediately before the eapol sequence.
cap2hccapx should use the following priority (from high to low) te get essid and essid_len:

1. re-associationrequest
2. associationrequest
3. proberesponse
4. directed proberequest (proberequest to mac_ap - not to BROADCAST)
5. beacon

Best regards
ZerBea