UPDATE: It appears that the bug has been fixed with iOS 15.1 / tvOS 15.1 / HomePod Software Version 15.1.
There is a workaround for the HomeKit shortcut bug in iOS 15 / tvOS 15. The problem occurs in shortcuts using for instance brightness percentages values (of lights) or brightness lux values (from sensors) in conditionals (if statements).
The workaround is getting the value using Get the state of Home, and then turning that value into a normal number using Get Numbers from Input. The resulting number can then be accessed either as a Magic Variable in the if statement, or you can make a variable from the number which can then be accessed in the if statement.
This is annoying, and time consuming for anyone with a lot of HomeKit automations using shortcuts, but at least it’s a way around the bug. Let’s hope Apple fixes the bug as soon as possible.