Tun0 not connect with right MTU

H’lo. I am connect to protonvpn via network manager (NetworkManager - ArchWiki). I set MTU to 1472, but network MTU is alway 1500. Why? Then (trick), I am use command “sudo ifconfig tun0 mtu 1472 up” at every startup. knock, knock. Is any way to fix? Please help