Serial3 on C10 doesn't seem to work.

With the SmartNixie tubes, I do the following:

Serial3.setup(115200,{tx:C10});
var nixie=require("SmartNixie").connect(Serial3,6);
nixie.setAllLED(128,128,128);
nixie.send() // nothing happens
// now, move wire from C10 to B10
Serial3.setup(115200,{tx:B10});
nixie.send() // it works

Not sure what's going on here.

Serial 4 on C10 works, as does Serial1 on B6 or A9, Serial3 on B10, and Serial5 on C12.

But serial 3 on C10 doesn't seem to work...