To reproduce: boot Fedora KDE live. Go to system settings, then the Keyboard page. Click the "Enable" slider next to Layouts. Add an English (US) layout. Add a Russian layout. Click Apply.

Now launch a console. Type a bunch of 'a' characters to verify that you see 'a'. Hit meta-alt-k to switch layouts, and immediately press 'a' a bunch more times.

For me, this produces a brief pop-up saying I've switched to the Russian layout, sometimes a brief 'permission required' dialog which disappears on its own accord, then usually just a *few* ф characters (the character you get when pressing the 'a' key with a Russian layout), then more 'a's. It seems like the switch works momentarily but then the system silently reverts to English (US).

If I flip the priority of the layouts, the effect reverses - we mostly get Cyrillic characters, pressing meta-alt-k briefly gives Latin characters but then we revert to Cyrillic. The tray indicator and overlay now get very confused and say we have US selected when we're actually typing Cyrillic. 

This seems to be specific to the live environment. On an installed system, things seem to behave correctly; switching layout is permanent.

The same behavior is observed if the switched configuration is produced by selecting Russian language in anaconda-webui rather than doing it manually in the system settings.

Filing against plasma-desktop as I've no idea what component is responsible for this.