Intermittent cursor visibility in NCURSES

Question

I'm teaching myself C (c17) and playing with ncurses.

I have written a function that clears the screen and draws a random number of colored period characters in random X, Y locations. (I've removed the color stuff in this code sample.)

I use curs_set(0) and 90-95% of the time this works correctly. Intermittently, though, the cursor remains visible.

I ssh to a Debian 11 server from both Windows 11 and a Mac Mini 2 and the problem is the same.

Any ideas?

Source that reproduces the problem:

#include <ncurses.h>
#include <stdlib.h>
#include <time.h>

int main(void) {
        initscr();
        curs_set(0);
        timeout(0);
        nodelay(stdscr, TRUE);
        start_color();
        bkgd(COLOR_PAIR(0));
        for (int i = 0; i < (rand() % (COLS * 2)) + COLS / 2; i++) {
                char ch;
                int x, y;
                do {
                        y = rand() % LINES;
                        x = rand() % COLS;
                        ch = mvinch(y, x) & A_CHARTEXT;
                } while (ch != ' ');
                curs_set(0);
                mvprintw(y, x, ".");
                curs_set(0);
        }
        curs_set(0);
        refresh();
        curs_set(0);
        struct timespec req = {2, 0};
        int result = nanosleep(&req, NULL);
        timeout(1);
        nodelay(stdscr, FALSE);
        clear();
        endwin();
        flushinp();
        return 0;
}