FLUSH: incorrect set of messages after flush

MultiplexerTest.testVirtualSynchrony() creates 2 MuxChannels, then sends 10 messages to each (so each should have 20 messages total).
The Channel.startFlush() is called, after 5 secs stopFlush() is called automatically. This returns true, an indication that flush succeeded.
However

• we see messages being received after flush has run
• we do not have the same and correct number of elements in both channels