This should be a comment but I am not allowed to comment due to my low reputation... Still thought this might help track down the issue.
I have a very similar but slightly different issue since I updated to Ubuntu 20.04. Alt + Shift combinations work on my built in keyboard but not on my external keyboard (Kinesis Freestyle 2, everything else works fine). Things worked fine under Ubuntu 18.04.
The out put of cat /etc/default/keyboard | grep -v ^# | sort -u reads as follows:
BACKSPACE="guess"
XKBLAYOUT="us,gr"
XKBMODEL="pc105"
XKBOPTIONS="grp:alt_shift_toggle,grp_led:scroll"
XKBVARIANT="altgr-intl,"
The output of lsusb reads as below
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 004: ID 138a:0097 Validity Sensors, Inc.
Bus 001 Device 003: ID 04ca:7067 Lite-On Technology Corp. Integrated Camera
Bus 001 Device 024: ID 8087:0a2b Intel Corp.
Bus 001 Device 022: ID 1c4f:0032 SiGma Micro Usb Mouse
Bus 001 Device 005: ID 056a:509f Wacom Co., Ltd Pen and multitouch sensor
Bus 001 Device 021: ID 058f:9410 Alcor Micro Corp. Keyboard
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Best Answer
As I suggested in Ubuntu 17.10 can't change the input switching shortcut to alt+shift :
You can achieve this using gsettings (command-line configuration tool).
Set forward switch to Shift+Alt(left)
Set backward switch to Alt+Shift(left)
To see the current setting value use get command:
Examples of other key bindings:
Changes will take effect immediately.