Additional potentially useful information (to be thorough). Program was compiled using the latest or near latest G++ (GCC) offered by the Arch Linux repos on Arch Linux. C++ was used when compiling. This bug occurs on the terminals xterm, urxvt, Alacritty, and MATE-Terminal.
The bug seems to not be related to terminal capabilities, as even on xterm which has no wide character/unicode support, the bad character printing can still be seen. A comment said it might have something to do with some character repeat control character, but I'd not have much insight into that.
Only other information I can think that could be important is the supporting code, in which I used the following function calls:
initscr(), setlocale(LC_ALL, ""), raw(), keypad(stdscr, TRUE), noecho(), and nodelay(stdscr, TRUE). The effect does not change if colour is enabled/disabled, or if I refrain from assigning attributes (colour or otherwise).