Following is a small gjs script to reproduce the problem.
Drag the window to the top using the title bar, release, the window gets maximized (mutter's behavior), then click into the window to unmaximize from the client.

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const Lang = imports.lang;
const Mainloop = imports.mainloop;
const Gettext = imports.gettext;
const _ = imports.gettext.gettext;

const Gio = imports.gi.Gio;
const GLib = imports.gi.GLib;
const Gdk = imports.gi.Gdk;
const Gtk = imports.gi.Gtk;

Gtk.init(null, null);

let win = new Gtk.Window();
win.resize(800, 600);
let ev = new Gtk.EventBox();
win.add(ev);

ev.connect('button-press-event', Lang.bind(this, function(widget, event) {
    let gdk_win = win.get_window();
    if (gdk_win.get_state() & Gdk.WindowState.MAXIMIZED)
        win.unmaximize();
    else
        win.maximize();
}));

win.show_all();
Gtk.main();

Created attachment 245461 [details] [review]
window: ensure tile_mode is coherent with requested unmaximize

The size of the test case window after unmaximizing as described in the reproduction steps is the same as it was prior to being maximized, so I suppose this was fixed at one point.