Not working for me either.
I know it used to work fine when buttons were just scripts (before the no-code functions were made available).
I suspect it may be because automations/buttons are being worked on, and perhaps the Fibery guys have broken something while adding new features.
It certainly looks like they have changed things that way.
I checked, and I still have some Types with the ‘old’ style action button scripting, and these work as expected (logging to the console works, and the pop-up on completion contains the return value).
It’s certainly frustrating to no longer be able to easily debug scripts, but I guess you could update a ‘scratchpad’ text field for the entity as a workaround.
It is executed on server-side and output will not be visible in browser. But thanks for request. Will think about possibility of viewing console in button log tab which will be available from button editor.
For now, as workaround you may use the way which was kindly proposed by Chr1sG above.