I've tried it again - I've stopped the VM, verified all related interfaces were gone from "ip a", started the VM again and again getting MTU 1500 (VM is connected to the vmbr0)
$ip a | grep 103
45: tap103i0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 9000 qdisc pfifo_fast master fwbr103i0...