Replacement SD card not mounted properly in device
Okay, I figured it out (hat tip to @IrfanLatif). What's going on is this:
Android focuses on one single &quot;primary&quot; SD card that can be used to permanently enhance internal storage, e.g. by moving apps to it.
When it is removed, the system remembers that apps and settings are still stored there, and waits that it shows up again. Other SD cards can be formatted for internal use, but they won't actually be usable until all ties with the known SD card are broken.
This means the card must be explicitly forgotten. The option to Forget a card is available from the Storage menu when that card is not currently inserted.

Be sure to backup everything you need before doing that.
Eject the card via the menu and remove it from the slot.
Tap the card's name in the storage menu and then &quot;Forget&quot;.
Confirm the warning that all data on that card will be lost forever, since the card is encrypted and Android is about to delete the encryption keys.

Only after that, Android will accept a different SD card as its primary storage and make it available to the rest of the system.