Ok, so I took an old sacrificial microSD card (4GB lol) and put it in a janky old USB reader I have, connected it to my Fedora PC.
I can reproduce what you are seeing if I delete the partitions off the card. Now, when I insert the reader, it blinks but nothing prompts me to format it or do anything with it. It does not appear in Dolphin in any way. Not an ideal user experience.
To do anything with it, I have to open KDE Partition Manager, find the device, and create a new partition on it. As soon as I do that, what you would normally expect to happen, happens: a popup announced it is available and gives me the option to mount it, and it shows up under removable devices in Dolphin.