When a Richtext Field is defined as part of a Composite Field within a Multivalue Field, using as transformers:

info.magnolia.ui.form.field.transformer.composite.NoOpCompositeTransformer
info.magnolia.ui.form.field.transformer.multi.MultiValueSubChildrenNodePropertiesTransformer

The Richtext Field disappears from the dialog when it is filled and after that the user is filling other fields and focus is lost (see support ticket to see the steps to follow to reproduce the issue).

In Browser developer tools we can see in 'Network tab' the requests, and it seems to be related to information lost, because we can see a the following info:

"state":
{
	"323": {   "errorMessage":"<div>Text field headline: This&#32;field&#32;is&#32;required&#46;</div>\n"  },
	"341": {   "errorMessage":"<div>Text field headline: This&#32;field&#32;is&#32;required&#46;</div>\n"  },
	"344": {
			"hideErrors":false,
			"errorMessage":null
		}
}, 

Attached you can find the requests using NoOpCompositeTransformer and MultiValueSubChildrenNodePropertiesTransformer, when the error comes up: requests_richText_disappears.txt

And requests using DelegatingCompositeFieldTransformer and DelegatingMultiValueTransformer , no error:
requests_richText_NO_disappears.txt