I think this is not a bug. The Command Syntax section of the pdksh manual says that the exit status of asynchronous lists is 0. (This agrees with POSIX.) Note the definitions of "command", "pipeline" and "list" in that section. Now the Job Control section says that jobs "are processes or groups of processes created for commands or pipelines", and "the processes of a job are placed in their own process group". So, pressing ^Z effectively turns 'echo one && sleep 10 && echo two' into 'echo one && { sleep 10 & } && echo two'.
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