Windows 10 command shell not running associated apps

Question

Recently the behaviour of my CMD shell (windows 10) changed, and I have no idea why.

Previously , when typing the name of a script xxx.py or blabla.txt would either run the script using python or open notepad with the text file. Lately, using the CMD shell, neither return anything, just the next prompt.

I created a test, doing the following

c:\> echo "Bla!" >> test.abc

go to the file in explorer, right click, associate 'test.abc' to notepad.exe then

c:\> test.abc 

does nothing - simply goes to next prompt, no error, no nothing. (c:\> notepad test.abc works)

however:

c:\> cmd /C test.abc

opens notepad as expected.

I tried the same on my tablet (windows 10) - and in there following those 3 steps above, both open the file properly in Notepad. (In both cases, the assoc/ftype and the xml created using DSIM exporting the defaultAppsAssociations etc show the same for my abc extension, so (I think) that is not the issue)

I compared user settings/privileges on both platforms, the cmd shortcut (%windir%\system32\cmd.exe) - etc., etc., seemingly no differences.

Although I can of course run my commands using 'cmd /C' and 'fix' it, problem is there are a load of batch files being useless at the moment - so rather then circumventing the issue, I prefer to understand and fix it.

I suspect a recent update or policy change might be responsible - but I have no idea what registry keys to look for.

Any ideas here as to why this particular CMD shell might refuse to execute the 'command' , rather than launching it with the appropriate app?

Identifying the relevant registry keys used by the shell would be a big help probably (why does 'cmd /c test.abc' manage?)