I'm seeing the same thing.
vrrp set on vlan interface.
physical inteface mtu is 10222
vlan interface mtu is 9170
vrrp interface mtu set to 9170 but actual mtu is 1500
this is on CCR-1072, RouterOS v6.42.11
config:
[admin@MikroTik1] > export compact
# jan/09/2019 19:30:00 by RouterOS 6.42.11
# software id = SYGW-LSV5
#
# model = CCR1072-1G-8S+
# serial number = XXXXXXXXXXXX
/interface ethernet
set [ find default-name=ether1 ] l2mtu=9116 mtu=9116
set [ find default-name=sfp-sfpplus1 ] l2mtu=10222 mtu=10222
set [ find default-name=sfp-sfpplus2 ] disabled=yes l2mtu=10222 mtu=10222
set [ find default-name=sfp-sfpplus3 ] disabled=yes l2mtu=10222 mtu=10222
set [ find default-name=sfp-sfpplus4 ] disabled=yes l2mtu=10222 mtu=10222
set [ find default-name=sfp-sfpplus5 ] disabled=yes l2mtu=10222 mtu=10222
set [ find default-name=sfp-sfpplus6 ] disabled=yes l2mtu=10222 mtu=10222
set [ find default-name=sfp-sfpplus7 ] disabled=yes l2mtu=10222 mtu=10222
set [ find default-name=sfp-sfpplus8 ] disabled=yes l2mtu=10222 mtu=10222
/interface vlan
add interface=sfp-sfpplus1 mtu=9170 name=vlan100-L3_MX960 vlan-id=100
/interface vrrp
add interface=vlan100-L3_MX960 interval=200ms mtu=9170 name=vrrp1 \
priority=101 vrid=101
/interface wireless security-profiles
set [ find default=yes ] supplicant-identity=MikroTik
/ip address
add address=10.10.7.5 interface=vrrp1 network=10.10.7.5
add address=10.10.7.2/29 interface=vlan100-L3_MX960 network=10.10.7.0
/ip route
add distance=1 gateway=10.10.7.1
/system clock
set time-zone-name=America/Chicago
/system identity
set name=MikroTik1
[admin@MikroTik1] >