Renamed the "eid" field to "id" in the LineItemForm form, since the problem is found. It was the "id" property of the enclosing FormField. If we extract the form from FormField, we can still access the "id" field.

tests/testlib_offset.py

@@ -66,7 +66,7 @@ class JournalEntryLineItemData:
                                 f"{prefix}-description": self.description,
                                 f"{prefix}-amount": str(self.amount)}
         if is_update and self.id != -1:
-            form[f"{prefix}-eid"] = str(self.id)
+            form[f"{prefix}-id"] = str(self.id)
         form[f"{prefix}-no"] = str(index) if self.no == -1 else str(self.no)
         if self.original_line_item is not None:
             assert self.original_line_item.id != -1