AU host: AUAudioUnit.parameterTree does not return for some AUs

I am writing an AudioUnit host, and using the AUv3 APIs. To obtain a list of AUParameters, I am retrieving the AUAudioUnit's parameterTree property, which is defined as an AUParameterTree. This works on some plugins (mostly Apple-provided ones), though when I try it with some third-party software instruments, the call does not return, and the debugger shows it getting stuck in what looks like C++ code for building a tree of parameters. The AUv2 APIs (using kAudioUnitProperty_ParameterList and kAudioUnitProperty_ParameterInfo) work correctly. (This is on macOS 10.14.)

What I'm doing looks more or less like:

Code Block swift
func parameterList(forUnit unit: AVAudioUnit) → 	[AUParameter] {
	return unit.parameterTree ?? []
}

Am I doing something wrong or neglecting some step?

Thanks