Samsung changed how the navigation bar works in Pie. You can still "hide" it, but it's more of a gesture now so that you swipe up at each spot to activate whatever that spot is for.
It no longer slides up the hidden bar where then you tap the button as it is for Oreo. It just stays hidden and you swipe up in the middle to close, swipe up on the bottom right to go back, swipe up on the bottom left for the recents list (I actually switched those around on my S8 and S9 to follow the standard Android layout). There's a faint bar along each spot on the bottom to let you know where to swipe. But, there's an option to turn that off if you want nothing along the bottom.
Go to "Settings" > "Display" > near the bottom "Navigation Bar" > "Navigation Type" section. And it shows you how it works. At the very bottom is the option to turn the "hints" off.
Takes a little getting used to, but because Google is trying to go all gestures in upcoming android versions, Samsung is just trying to get ahead of it.