If you want to use custom properties, try something like this
Seems to work
Option Explicit
Private Sub OK_Click()
ThisDocument.CustomDocumentProperties("SomeProp").Value = Me.custProp.Value
'https://stackoverflow.com/questions/54769648/custom-document-property-not-getting-saved-in-word-document
ThisDocument.Saved = False ' <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Me.Hide
Unload UserForm1
End Sub
Private Sub UserForm_Initialize()
Dim oCP As DocumentProperty
Dim i As Long
With ThisDocument
For i = 1 To .CustomDocumentProperties.Count
If .CustomDocumentProperties(i).Name = "SomeProp" Then
Set oCP = .CustomDocumentProperties(i)
Exit For
End If
Next i
If oCP Is Nothing Then ' custom prop doesn't exist yet
.CustomDocumentProperties.Add Name:="SomeProp", LinkToContent:=False, Value:="", Type:=msoPropertyTypeString
Else
Me.custProp.Value = oCP.Value
End If
End With
End Sub