When you formatted that microSD card as Internal that changed the card significantly -- its file system was changed from being FAT32 to ext4, and it was encrypted so it only works with your specific phone. At that point you no longer should have used that card independently the way you did previously. The card's storage media was essentially and directly merged with your phone's internal storage. Your phone's internal storage is also using ext4 as its file system, and is encrypted. Also at that point both that card's storage media and your phone's storage media are using the same file system. You don't interact with the card any longer, it's managed and maintained by your phone.
By flip-flopping on changing that card back to portable, you are intentionally creating an instability issue with your phone. Its installed Android operating system was using that combined storage media as one expanded volume. Now you're going to force to be reduced is a major way. So just be prepared for a few reboots as the OS is being forced to readjust its fundamental storage capacity, and possibly needing to reinstall a few apps to restore them back to working condition. With the card being set as Internal, it cannot be reused in any other device, just to emphasize again, its encrypted to only work with that particular phone.
Anyway, if you do want to reset the card back to Portable, be sure to do the process using the Settings >> Storage menu. You're making a big change to your phone's functionality so be sure to follow the prompted warnings and instructions. Really, DO NOT just remove the card and reformat it a different device. So do the deed if you must, but format the card back to Portable using your phone. Once back to Portable , that returns it back to using FAT32 and it removes the encryption so you can reuse the card elsewhere again.
.
