I have a custom button that I place in the ribbon menu but what I observe is that sometimes the button is visible and sometimes not, it depends on the size of the explorer and compose windows. If you resize to a smaller size it is not visible and if you resize to a bigger size it is visible. It only happens in simplified view but not in classic. So in simplified view it seems Outlook decides which buttons are being shown and which not based on a criteria that I don't know, maybe on the space available in the ribbon menu which in turn depends on the size of the window?
Anyway, If I click on commands bar button ("..." three dots button) at the end of the ribbon menu and then from that menu I do a mouse right click on my button and select "Pin to ribbon" for it, then my button is always visible in the ribbon menu regardless of if the view is classic or simpified or even if window is resized to any size.
Is there any way programmatically to indicate Outlook to always show my button in the ribbon menu?
No, the Outlook extensibility model (nor the Fluent UI) doesn't provide anything for that. You may try using RegMon for Windows to track windows registry changes in case if Outlook keeps such preferences there.