I have several instances of dashboards where I included notes, descriptions, text boxes, etc. and have occasionally selected a color for the text only to change my mind. Once you change it, it seems there's no going back? I selected black, but this does not change to a readable color when I switch to dark mode so I'm stuck.
This sounds like a bug that should be raised with Client Care, so that they can get the Analytics team to fix it.
Can you try duplicating the visualization (as in the text component) in the same panel and see if the behavior changes?