I've test it in Build 19238 and 19578, all nodes at every level are missing children nodes. Like this:
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Thanks for pointing this out. This should be fixed in 2019.1 beta b19604 and will be in the 2019.1 release. Our apologies this took so long to deal with. Part of the complexity is that this is a write only format that is used within a complex third party system (ArcPro scene layers) so it can be hard to isolate and reproduce problems. Still, we will work to improve our automated testing so the risk of something like this happening going forwards is reduced.