I have just switched to Android Pie. When I press the key “U” and hold it, Smart Keyboard Pro pops up a small window where I can choose secondary functions, e.g. 7 oder U with accents and umlaut.
However, this popup not placed correctly anymore. It gets hidden behind the current app’s main window. Thus, I cannot type certain numbers anymore without switching to the number mode. It all worked fine up to Android Oreo 8.1.