I have the same issue. Hey Opera people, listen to us! Please disable OSK if the system has physical keyboard. Earlier it was working exactly this way.

By now, there is a workaround. You will need a Microsoft Devcon script.

1. Create a folder, let's say C:\touch
2. Download devcon from Microsoft: https://docs.microsoft.com/en-us/windows-hardware/drivers/devtest/devcon
3. Create a *.bat file in \touch folder:
   set "touchscreenid=HID\VID_03EB&PID_8204&MI_00"
   devcon status "%touchscreenid%" | findstr "running"
   if %errorlevel% == 0 (
   devcon disable "%touchscreenid%"
   ) else (
   devcon enable "%touchscreenid%"
   )
4. Please note that HID\VID_03EB&PID_8204&MI_00 is the HID signature of my own Toshiba touchscreen. Most likely yours will have different HID! You can fetch your touchscreen HID from Device Manager tool and then replace this value in the script above.
5. Now, create a shortcut to the script.bat file with the script:
   C:\Windows\System32\cmd.exe /C "C:\touch\script.bat"
6. Click on that shortcut each time you are connected to the physical keyboard and when you're about to disconnect.

Of course this shortcut-clicking thing is annoying as ***k, but at least it will help you while Opera people are working on the regular fix.