I have also consistently had the same problem. In R2024a and earlier I had a few of my commands on the quick access toolbar and I think they are still there in R2025b (they are still under 'Favorites' so they exist), but the toolbar itself is almost never there. I use the undo and redo buttons a lot (especially redo as I don't remember the keyboard shortcut!), which are on that toolbar, but I can't because the whole bar just keeps disappearing.
Right now the bar is there on my Editor window, but most of the time I find it has gone, and also isn't in my App Designer window (nor the main Matlab window).
So unless I am misunderstanding the original question, it isn't about how to add something to the quick access toolbar it is the toolbar itself that keeps disappearing, even with its default buttons.
