Excel UDF Output is formatted as stale values

Question

I have some C++ UDF developed to extend Excel functionality. With the latest Office365 version I am seeing the results from these UDFs flagged as stale data (strike through formatting). Is there a mechanism for indicating that the results from the UDF are up to date. I know how to disable this new Excel capability but I do see it as valuable for other data so would like to keep it switched on.

I am exposing the UDFs through the xlw 5 toolkit which runs over the xlCall32 dll as the interfacing library.

Thanks for any information you can provide.

Regards
Peter C