The Terminal App can't zoom to maximum. It can't zoom to the bottom. See the pictures. The last picture shows that TextEdit app can be zoomed to the bottom, but the terminal app can't.
Unlike TextEdit, Terminal zooms in discrete steps, determined by the current line height.
That is, when you drag the lower "border" of the Terminal window to the bottom edge of the screen, it will resize in steps that allow the full line height to be displayed. This prevents that the top of the upper line is cut off and thus increases readability.
After upgrading to Catalina, I found a strange behavior.
I may be wrong, but I don't think this behavior is new to macOS 10.15 "Catalina", and it doesn't seem to be configurable.
Did you do that by dragging the bottom edge, alt-clicking the green dot, or the bottom right corner? – Tetsujin – 2019-11-28T13:52:05.680
I did all of these, but there is always a gap in the bottom. – Gary Allen – 2019-11-29T01:12:50.143