18.2 bug stops complex HomeKit automations from working

Do you have some HomeKit automations set up that suddenly stopped working after your Home hub (HomePod or Apple TV) updated to 18.2, and you're losing the will to live trying to diagnose what happened? It's not you, it's an insidious and obscure bug in 18.2. If you have automations that do not only set scenes or accessories but are shortcuts, and these shortcuts contain the 'Calculate' action, this action will fail. It works if you test the automation in the Home app, whether step by step in the godawful shortcut designer (with the 'play' button) or if you just tap 'Test Automation', but it silently crashes if the automation triggers normally.

I reported this bug with the Feedback app during the 18.2 beta cycle, but it nonetheless made it to the final release. It's obscure and probably doesn't affect that many people, so who knows when it'll get fixed - meanwhile the only solution I found is to downgrade a HomePod to 18.1 and hope it stays there.