It looks like Xcode 13 introduced yet another bug related to AppleScriptObjC: script classes are not recognized automatically in Interface Builder. (That’s in addition to a more serious bug described here: Xcode 13 doesn't recognize outlets in AppleScriptObjC scripts)
I’m referring to the class names we specify in
script ASTestClass property parent : class "NSObject" <...> end script
The problem happens when you add an ASOC class object to Interface Builder, on attempt to assign the class name to the object.
Normally, when you start typing the class name in the Identity Inspector, it should be automatically recognized and suggested in auto-completion. It worked in all previous versions. Not anymore.
You’ll have to type the exact class name manually.
The bug appears to be only cosmetic: after typing the class name manually, the object behaves as expected.
This is yet another bug that was introduced somewhere after Xcode 13 beta 1, as this bug isn’t present in beta 1.
P.S. It appears that after beta 1, the development of Xcode 13 went downhill in general. Another major bug was introduced in later betas: slow typing that virtually rendered Xcode 13 useless for many users. It was only fixed recently in Xcode 13.2.