EDIT#2: I think I found the cause of the issue: in /proc/net/vlan/config there is an old entry for vlan620 which links to bond0 (I was trying to do bonding originally) plus an old vlan620 file again tied to bond0.
iface eth0 inet static
up ip addr add xx.xxx.252.35/24 dev eth0 label eth0:0
down ip addr del xx.xxx.252.35/24 dev eth0 label eth0:0
up ip addr add xx.xxx.252.34/24 dev eth0 label eth0:1
down ip addr del xx.xxx.252.34/24 dev eth0 label e