It's been a while since I looked at the file, but IIRC the AR is fine but the letterboxing has been cropped - most hardware players won't know what to do with the non-standard resolution so they stretch out the existing image to fit 16:9.
UncropMKV will sort this out - it restores the image to true 16x9 (letterboxed, obv.)
Rob, I need to send you a long email! Speak later.