The button should be there, in the bottom right corner. Could you please provide your iOS version, device model and maybe a screenshot of a screen where you expect the button to show up, just in case I notice something?
I think I know what is happening, the app is bigger than the viewport, and the top right buttons like search and the bottom right buttons like add are out of the screen, if I resize the window then it sort of readjusts but not completely right.
I don’t know what I did before that it fit properly, but then I closed and opened the app and had the same issue.
Os: 16.5.1 (c)
You can see here the buttons disappeared, you can see a bit of the search icon above.
After resizing the icon comes back but it is not properly positioned and it cuts off.
If you can please remove the screenshots once no longer useful. Not a lot of personal information in them but I rather not have them public