I did just find this thread, and saw that for this person, switching to pressing “ctrl + shift + .” worked on their Windows machine rather than the normal “ctrl + .”.
Even though “cmd + .” doesn’t work for me, when I tried “cmd + shift + .”, it works! I’ll just use this from now on.
Like the author of that thread, I too have other languages inputs that I can switch between, so perhaps that has something to do with the cause. I did look at my language keybindings, and none of them are using “cmd + .”. And I also have not changed any of my language settings for years. This problem occurred for me all of a sudden, but I wonder if it happened after some MacOS update.