Roberto, I figure out how to trigger and disable the problem and I think it's a BUG.
The problem occurs when I press SHIFT + SPACE (together).
I went though the list of shortcuts in the Gnome Control Center under Devices ->Keyboard. No shortcut is set for SHIFT + SPACE
As I told you before, Input Source Options is set to: "Use the same source for all windows"
But the shortcuts to change input, in case it was set otherwise are: SUPER + SPACE and SHIFT + SUPER + SPACE.
Before yesterday when I typed SHIFT + SPACE I'd got a space character, now I have this weird behavior that I don't even know what it means.
Thank you again for your help.
Regards.