It looks like there’s a bug in the threeState functionality since webix version 9.0.
I added threeState: true to an existing sample:
https://snippet.webix.com/u9dw5b73
After checking/unchecking Music all children get checked/unchecked as expected. However, when I check Music and then uncheck for example Jimi Hendrix, the parent disappears. Checking the Jimi Hendrix checkbox makes the parent reappear.
This only happens when using webix version 9.0 or higher in the code snippet. Version 8 seems to work as expected.