SSL Keystore ignored after modifying SOAP

I've been using SOAPUI to test messages within a private WAN.  Recently we've had a need to for an internet-facing endpoint so the server administrator implemented security certs.  I've been provided with a keystore file, which I add to my project under WS-Security Configurations > Keystores, then I specify it as the SSL Keystore property in the request properties.  This works correctly until I edit the SOAP message; the server replies informing me that a SSL Certificate is required.  If I close and re-open SOAPUI and re-send the edited message it will succeed again.

In essence, changing the SOAP message seems to omit the selected SSL Keystore until I re-load SOAPUI.  I downloaded a trial of ReadyAPI which exhibits the same behaviour.

SoapUI 5.3.0 on OSX El Capitan 10.11.6