A brief moment of bad OPC quality is normal following an application download to an AC 800M.
If the application is large, the time to reload the OPC server's map of controller tags may take a bit longer.
The session.log of the AC 800M OPC server is of interest here as there could be other reasons for lack of quality.
My bet though, is that you have a larger application and your customer is concerned about red crosses at download. A handful of seconds is "normal". A large application can take a bit longer to refresh.
And how do you get it work again? As far as I know, if you use CCB (not CBM) with 800xA, you have to copy the .cdoXml, .rrsXml and .ctdXml files manually from the CCB path to OPC server path after each download.