Ah, the joys of waking up to find the Mac's done an overnight upgrade… and erm, suddenly things stop working. Thankfully, me and Claude managed to work out what the fuck is going on… I'm sharing here, as well as having raised in on https://feedbackassistant.apple.com/feedback/22280434 (that seems to need a login?).
Product: macOS 26.3.1 (Darwin 25.3.0, Build 25D771280a)
Component: Networking → DNS / mDNSResponder
Regression from: macOS 25.x 26.3.0 (working immediately prior to overnight update)