Sail failing on new conformance test

[robmoffat]

Bug Report

Steps to Reproduce:

We're running this test: https://github.com/finos/FDC3/blob/conformance2.0-rm/toolbox/fdc3-conformance/User-Channel-Tests.md UCFilteredUsage3 and UCFilteredUsage4 using the https://github.com/finos/FDC3-conformance-framework/tree/2.0-release-candidate-2 branch of conformance tests

Expected Result:

When an app broadcasts on a user channel, then another app subscribes, the second app should receive the context from the first app, even if it subscribes after the broadcast has finished.

Actual Result:

Doesn't seem to happen

Environment:

HEAD of Sail