18323 may not be new enough (that number means the 323 day of 2018). Let’s try getting the next one before digging in too deep.
@archinate1 I just pushed Rhino3dmIO.Desktop v6.11.18325.15581-dujour (daily build from our 6.11 branch); this one definitely contains Steve’s fix 
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@will and @stevebaer Got it! Feels rock solid now! Been working with InstanceDefinitionGeometry like a madman this morning! 
Thanks!
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Whew, that’s a relief 
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