Keynote custom color swatches reset every time I close the app

Hello, I am happy to write in this community where I hope to find the support I am looking for.

For some time now I have had a strange problem with colour management in keynote. The custom colours that I normally save by dragging and dropping colour swatches into the grid (see attached image), simply auto-delete each time I restart Keynote. Previously these colour swatches would remain saved in memory until I decided to delete them, and moreover they were available in all Apple software: now none of this happens anymore.

I suspect it is a bug related to my computer as I have asked some colleagues and for them everything is normal, but I don't know how to solve it so I am asking for help.

Thanks in advance.

Alessandro