Setting working directory after fork means there is a race with pane_current_path (especially on platforms with systemd which have to take time to do some additional faffing around). To avoid this, change it before fork and back in the parent afterwards. GitHub issue 4719.

usr.bin/tmux/spawn.c

···
-/* $OpenBSD: spawn.c,v 1.34 2025/04/24 08:55:40 nicm Exp $ */
+/* $OpenBSD: spawn.c,v 1.35 2025/12/08 08:04:35 nicm Exp $ */
 
 /*
  * Copyright (c) 2019 Nicholas Marriott <[e-mail]>
···
 	struct environ		 *child;
 	struct environ_entry	 *ee;
 	char			**argv, *cp, **argvp, *argv0, *cwd, *new_cwd;
-	const char		 *cmd, *tmp;
+	char			  path[PATH_MAX];
+	const char		 *cmd, *tmp, *home = find_home();
+	const char		 *actual_cwd = NULL;
 	int			  argc;
 	u_int			  idx;
 	struct termios		  now;
···
 		goto complete;
 	}
 
+    /* Store current working directory and change to new one. */
+	if (getcwd(path, sizeof path) != NULL) {
+		if (chdir(new_wp->cwd) == 0)
+			actual_cwd = new_wp->cwd;
+		else if (home != NULL && chdir(home) == 0)
+			actual_cwd = home;
+		else if (chdir("/") == 0)
+			actual_cwd = "/";
+	}
+
 	/* Fork the new process. */
 	new_wp->pid = fdforkpty(ptm_fd, &new_wp->fd, new_wp->tty, NULL, &ws);
 	if (new_wp->pid == -1) {
···
 		return (NULL);
 	}
 
-	/* In the parent process, everything is done now. */
-	if (new_wp->pid != 0)
+	/*
+	 * In the parent process, everything is done now. Change the working
+	 * directory back.
+	 */
+	if (new_wp->pid != 0) {
+		if (actual_cwd != NULL &&
+		    chdir(path) != 0 &&
+		    (home == NULL || chdir(home) != 0))
+			chdir("/");
 		goto complete;
+	}
 
 	/*
-	 * Child process. Change to the working directory or home if that
-	 * fails.
+	 * Child process. Set PWD to the working directory.
 	 */
-	if (chdir(new_wp->cwd) == 0)
-		environ_set(child, "PWD", 0, "%s", new_wp->cwd);
-	else if ((tmp = find_home()) != NULL && chdir(tmp) == 0)
-		environ_set(child, "PWD", 0, "%s", tmp);
-	else if (chdir("/") == 0)
-		environ_set(child, "PWD", 0, "/");
-	else
-		fatal("chdir failed");
+	if (actual_cwd != NULL)
+		environ_set(child, "PWD", 0, "%s", actual_cwd);
 
 	/*
 	 * Update terminal escape characters from the session if available and