Ctrl+Shift working but not Shift+Ctrl in Ubuntu 20.04 LTS
You might have Locate Pointer setting turned on in Universal Access under Pointing & Clicking section. Turn it off to see if it works.
I had the same issue and was confused for some time. Searching online and found your question. Only until then I realized what may have caused it. There must be a bug in that Locate Pointer functionality which hijacked the Ctrl key event in some way.