Window in focus doesn't minimize when I click on its icon in the taskbar (Windows 10)

It is unlikely settings were changed, by you or by an update. This behavior, AFAIK, is not something you can alter or customize, even with registry edits.

This is likely a temporary bug. Try to gracefully restart Explorer.exe to solve the problem without a reboot.

Open up a command window with administrative privileges

Paste each line below, one at a time, followed by enter

taskkill /f /IM explorer.exe
explorer.exe
exit

If this doesn't solve the problem, try rebooting.

If rebooting doesn't handle it, use system restore to bring the system back to a date prior to the problem start date.

Open the Task Manager (by hitting Ctrl+Shift+Esc or any of the other methods).  On the "Processes" tab, scroll down to "Windows Explorer" and highlight it.  Click the "Restart" button in the bottom right corner of the Task Manager.  Applications like Windows Media Player will be working as full screen after this change too.