Can you post the custom property code? Especially the part where you save the property data.

I' m thinking to a permission for editor problem. Seem that editor has not permission on the legacy page which used to display keywords data.

Sorry my late answer. I think problem is not in legacy page because control is made as dynamically in CreateEditControls. EPi7 will put that control in legacy editor-box.

Keywords are saved in ApplyEditChanges method.

Here is some of the code:

public class PropertyKeywordsControl : PropertyDataControl
{
 System.Web.UI.WebControls.ListBox lBox;

public override void ApplyEditChanges() // saves property value
{
 string strKeywords = "";

 foreach (int i in lBox.GetSelectedIndices())
 {
  strKeywords += lBox.Items[i].Text + ", ";
 }

 // remove ", " from end of the string
 if (strKeywords.Length > 1)
  strKeywords = strKeywords.Remove(strKeywords.Length - 2, 2); 

 // Turns keyword list as comma separated GUID string.
 var keywords = CommaSeparatedStringHelper.ParseKeywordsToGUIDs(strKeywords);
 
 // save GUID list as Keywords custom property object
 var propertyValue = new Keywords();
 foreach (var keyword in keywords)
 {
  propertyValue.Add(keyword);
 }
 SetValue(propertyValue);
}

// Edit controls which will be presented in legacy editor - box
public override void CreateEditControls()
{
 lBox = new ListBox();
 lBox.Rows = 13;
 lBox.SelectionMode = ListSelectionMode.Multiple;

 // Get list of all keywords to listbox
 SubjectWS sWS = new SubjectWS();
 lBox.DataSource = sWS.ListOfKeywords();
 lBox.DataBind();

 // mark already selected keywords in listbox
 var keywords = PropertyData.Value as Keywords;
 string strKeywordlist = CommaSeparatedStringHelper.ConcatenateAsiasana(keywords) + ",";
 foreach (ListItem itmKeyword in lBox.Items)
  if (strKeywordlist.Contains(itmKeyword + ","))
   itmKeyword.Selected = true;
 
 // add listbox to editor view (in this case in legacy editor)
 this.Controls.Add(lBox);
 base.CreateEditControls();
}

. . .
}
  
public class Keywords : IEnumerable<string>, IReadOnly<Keywords>
{
// Keyword structure are pretty similar as this code example:
// http://joelabrahamsson.com/creating-a-custom-episerver-property-with-a-custom-class-as-value/
. . .
public Keywords()
{
 values = new List<string>();
}

public Keywords(string strKeywords)
{
 values = CommaSeparatedStringHelper.Parse(strKeywords);
}

public static List<string> ParseKeywordsToGUIDs(string values)
{
 // gets keyword GUIDs from cache and returns comma separated list of GUIDs
}
. . .
}