How to manually mount internal (adopted) SD card in Android 6?
I had the same problem and did the following (you can try upfront whether step 7 does it already for you):

Backup old card (whole disk) as an image using ddrescue, save decryption key on your computer

Remove adopted card

Take a new card, format as internal, migrate data to this card, save decryption-Key on your computer, shutdown phone, remove new card

mount image and new sdcard unencrypted to Folder A and B

rsync A -&gt; B

new card into phone, start debugging

Reset SELinux permissions using
restorecon -R -v /mnt/expand/[yourpartition]/*


as root in adb shell.
This did the trick for me. The new card did not vanish anymore and keeps persistently mounted.