I tested in Linux Mint 22.1 Cinnamon (X11) with the deb packages of Opera Stable 125.0.5729.15 and Opera Developer 126.0.5742.0. I can indeed confirm that the --window-position and --window-size command-line arguments are indeed ignored by Opera and work fine in Chrome.
Indeed seems like a bug to me. Filed RNA-1806 as I don't think the issue is known (except by us here).