File access failure on encrypted SD card


I am assisting with Beta testing on a Podcast App and we are struggling to track down a file permissions issue. It occurs only on External Storage, which is a micro SD card. When the card is encrypted, the App loses access to the downloaded mp3 files: but only after the App updates. Although internal storage is also encrypted, the App does not lose access. I don't develop on Android but this looks like something an Embedded 'C' OS would do. It behaves like a security feature where Android recognises the CRC of the App has changed, and as a result blocks access to shared files.

Is this a feature of Android 6.01? Or potentially a behaviour of Samsung Smart Manager virus protection? Phone is A3 2016.

If I decrypt the card, it works. If I move default storage to the encrypted phone, it works. If I re-download the mp3, it works uptil the next App update.

Thanks for the help!