Just tinkering around, and found that I can place water from a bucket in front of a piston, but water source blocks won’t ever flow back to fill the void left by the retracting piston. I’m suspecting something amiss in mesecons. Perhaps the water source destructor isn’t being called, and that’s not updating adjacent node meta data? [shrug]
I’m not going to chase this one down right now. Noting it here so it doesn’t get forgotten.