How to reproduce

• Declare a metadata with a right:
  read-write-direction="read"
  for example

  <cms:metadata name="alt-textbis" type="string">
  			<label i18n="true">CONTENT_NEWS_IMAGE_ALT</label>
  			<description i18n="true">CONTENT_NEWS_IMAGE_ALT_DESC</description>
  			<restrict-to>
  				<right read-write-direction="read" id="Thumbnail_Vertical_Rights"/>
  			</restrict-to>
  		</cms:metadata>
  

• edit the content with the right and set something, save
• modify your right
• edit the content, the field is not displayed, save
• restore your right
• edit the content

Actual behavior

• the content of the field is lost