Problem
This only affects the Workstation and KDE live images. If you boot one of these, run the installer, configure multiple keyboard layouts (or multiple layouts are automatically configured for the language you chose), and then switch to a secondary layout using the integrated layout switcher, the keyboard layout will switch back to the primary one whenever you press a modifier key (e.g. shift, alt or ctrl).
This isn’t usually a problem on Workstation, because there are few places where you are likely to use the secondary layout for typing characters. However, for KDE, it’s more likely, because user creation is available, and you might want to use the secondary layout to type the user’s real name.
Related Issues
Bugzilla report: #2016613
Workarounds
One way to avoid dealing with the problem is just to not use modified characters from the secondary layout during installation; use some other character temporarily, then change it after installation.
If you do want to enter modified characters using the secondary layout during installation, you have two options. One is to select the primary layout, then hold down the modifier key and keep it held down while selecting the secondary layout; you can now type as many modified characters as you like until you release the modifier key (whereupon the primary layout will immediately become active again). Otherwise, you can log out from the live session, and on the login screen, select “Desktop Session: Plasma (X11)” (KDE) or “GNOME on X.org” (Workstation) at the very bottom of the login screen, and then login again as the “Live System User” (no password). With an X.org session, this bug will not occur.