I wish I could help you but I too am in the dark as to how variables are pass back and forth. Given the “Test set variable” process and the “Test set variable” step have the same name, have you tried giving them different names? I have no idea if AppSheet is trying to call [Test] from the step or the process.
Not to hijack your thread but to answer a question with another question, do you have any knowledge as to how AppSheet would read in the ID valuable you are passing into the “Test set variable” process from the process side? Would that be [Test set variable].[ID] given you named the step the same as the process?
I hope I am of help and not just adding to the confusion.