Thanks for the detailed info.
I've installed the latest Ardour 6 pre build (pre1.156) and can play Zebra just fine (that's in UbuntuStudio 17.04 though).
Not really sure what to do, but since it works in Reaper, and since there've been bugs in Ardour which affected our plugins in the past, I'd suggest to file a bug report with them to see if they can find out what's happening.
Sorry that I can't be more of a help.