Internal Error on Rectangle Tracking under iOS 11

I am using a VNSequenceRequestHandler to track rectangles detected by VNDetectTextRectanglesRequest s. This works flawlessly under iOS 12, however under iOS 11.4.1 the tracking fails.

I see two error messages that are logged each time I attempt to track, at every frame.

Error: VNRectangleObservation object is expected to initialize Rectangle Tracker
Error: No objects to track passed to the tracker

As the perform method on the VNSequenceRequestHandler can throw, this is the error returned by the method:

Error Domain=com.apple.vis Code=14 "No objects to track passed to the tracker" UserInfo={NSLocalizedDescription=No objects to track passed to the tracker}

Since this works under iOS 12 (with no code changes), I assume there has been a bug fix or extension of the internal functionality to make this work there.

Is there anything I can do to make this work under iOS 11 or is this more of an unsupported thing?