Comment 3 for bug 1722540

If you want an ao record to stay invalid after processing when nothing explicitly sets its VAL field you can set that VAL field to NaN in the DB file. We could change the default value of VAL to be NaN but I'm not sure if that would be a good idea at this point.

Looking briefly at the code I suspect this is happening because even if nothing writes a new VAL the record still has to call convert() to handle an OVAL value changing due to OROC.

Maybe this is a "feature" of the ao record that just needs to be documented (like the HIGH field of the bo record which is similarly unique), with "set VAL to NaN" given as a workaround if you prefer the UDF behavior.