I don't know how I missed this before! The Test Tool has a default setting of "Don't show errors if there are other errors in the workflow". And it's easy to overlook the tiny print at the bottom of the test tool configuration window! Sounds dangerous to me; I wouldn't want to silence errors unintentionally. Now EVERYTIME I use a test tool, I uncheck this.
There's an idea accepting votes to change this default setting: https://community.alteryx.com/t5/Alteryx-Designer-Desktop-Ideas/Test-tool-Change-default-on-error-re...
This is already the default in Alteryx Deisgner version 2023.1 - as, I agree, it should be.