I’m facing a possible bug in the new Noes React UI.
Basically validation, at least with NotEmptyValidator, is applied only after the first modification of the mandatory property.
Steps to reproduce:
Have a NodeType with a property with:
properties: [PROPERTY_NAME]: [...] validation: 'Neos.Neos/Validation/NotEmptyValidator': 
Apply values to properties leaving the mandatory property blank -> modifications get applied without warnings or “failed validation” feedback
Fill in mandatory property and Apply
Empty out mandatory property -> the “validation failed” popup is shwon
Imho it looks like a bug cause I would expect the validation to kick in before the first apply if fails.
A possible work-around is to use defaultValue, but it’s a work-around
If anyone can confirm this behaviour I’d gladly open an issue on github. I already looked for openones without finding anything (let’s hope I did not miss it).