It is impossible to unmount removable device's partition without removing device.
I was experimenting with preparing bootable usb stick, so I constantly needed mount/unmount partitions on usb stick to check bootability with qemu. It is convenient to mount a partition by just clicking to it, but the problem is that when you want to unmount it, you only have Safely Remove action. I do not know how this works internally (probably uses something like eject function of a usb device), but when you use it, device disappears from Dolphin, and next time you want to mount it, you should physically replug usb stick to your pc.

Is there a way how I can again "activate" usb stick to be shown in Dolphin without replugging physically?

I have noticed that for partitions of internal devices another function is used, which is called Unmount. Can you please implement it also for removable devices?

Workaround 1:
unmount via terminal. But it becomes uncomfortable.
Workaround 2:
add a second partition to usb stick and never mount it. When you click Safely Remove on a first partition, then both partitions disappears, but then appears again after a second. 

Operating System: Arch Linux 
KDE Plasma Version: 5.15.3
KDE Frameworks Version: 5.56.0
Qt Version: 5.12.2
Kernel Version: 5.0.4-arch1-1-ARCH