When you change a microSD card from its Portable status to Internal, that actually involves a pretty profound change to the card itself and how the installed Android operating system in your phone uses it. This also involves a number of added variables that have to assessed.
Typically a microSD card is formatted as either FAT32 or exFAT, That allows other devices, like desktop PCs, laptops, other mobile devices, webcams, etc., to all be able to mount and utilize that card given other operating systems also include cursory support for FAT-based file systems (an antiquated, proprietary standard that Microsoft stopped supporting decades ago but still retains its licensing). Anyway, that's the portable aspect for a card that's still Portable. When you reformat the card to be Internal, all data gets wiped, the file system gets changed to ext4, and it gets encrypted with the encryption key tying the card only to the mobile device that reformatted it. That's an important aspect.
When a card is changed to Internal, its file system and the encryption are now similar to the file system in the phone, which by default is already using ext4 as its encrypted file system. Both are now in parity, and as Internal its now a matter where the installed Android file system now considers that card's storage media to be merged to the phone's internal storage media. Instead of two separate storage media volumes when Portable, it's at that point just one, now increased storage media volume when Internal. That's why you shouldn't remove a microSD card when its set up to be Internal. The installed Android OS views the card to now be an internal component, so taking it out like when it was Portable isn't advisable. Besides there isn't much you can do with it anyway, MS Windows nor Mac include support for ext4 and the card is now encrypted so the card won't be readable even if you manually install a utility to include ext file system support. If in the future you opt to return the card back to Portable, you can use your phone's Settings >> Storage menu to reformat it to Portable, but its something of a drastic change again so be sure to read through all the warning popups about data loss.
So all that's just the fundamentals on Portable or Internal. The issue you're probably now dealing with it likely due to differences between the Settings menu and installed apps. And that brings up an important question, how are you determining your phone is out of storage space but still shows 20GB free?
Keep in mind that the Settings menu will show you a combined internal storage media and external storage media amount. There will be two volumes but shown as one.
Conversely, something like a file manager app, will show you the single, combined storage media amount. A file manager app can no longer distinguish between the internal storage and the card storage, it can only detect and show you the now virtually combined storage.
-- So what device do you have, and which version of Android is it running?
-- How big is the internal storage (i.e. 32GB, 64GB, 128GB) and the microSD card?
-- How are you determining that 20GB free amount? The Settings >> Storage menu? A different utility in the Settings menu? An installed app and which is it?
