If it's not too late to investigate for this update, I noticed that Pd 0.55-0 suddenly stopped supporting JACK with 8+ output channels. When I set output channels to 8 or more, Pd freezes and I have to issue a "kill -9" for the process. This configuration still works for me using Pd 0.54-1, where I regularly use 20 output channels. Here's the software/hardware situation where I'm experiencing the problem in case anyone else can reproduce it:
I can post an issue on the github repo if this is something that needs to wait until later...
William