Here is (I think) a recipe to demonstrate what the OP has in mind
(tested with xterm, rxvt, st, kitty, alacritty, Linux console):
1. emacs -Q -nw
2a. if your terminal emulator has a light background: M-x load-theme
RET modus-vivendi RET
2b. if your terminal emulator has a dark background: M-x load-theme RET
modus-operandi RET
Observe at that point that the last character (the one at the bottom
right) does not have the background color of the chosen theme. So far
so good.
The OP filed a bug for this, and he might work on it he said.
This is because 'write' deliberatly leaves the bottom-right corner alone
if the terminal has "auto right margin" (autowrap).