I hope you have the "accpet-data" knob under the VRRP config to allow it to respond to ICMP traffic.
Apart from VRRP config I hope you have the interface and prefix defined under "protocols router-advertisement" and "show ivp6 neighbors" is showing the MLX as neighbor.
As usual, do check the firewall filter.
Here is the configuration example for VRRPv6
If this doesn't help send me the config, show ipv6 neighbor, show vrrp output to look into it.
PS: Please accept my response as solution if it answers your query, kudos are appreciated too!
It seems like split brain situation. Your MX router is the showing as VRRP master and you have learned the VMAC from other router too but unfortunately its in incomplete state. In working scenario it should in reachable state.
2a01:xxx:xxxx:xxx::4 00:00:5e:00:02:01 incomplete 0 no no ae16.0
fe80::1 00:00:5e:00:02:01 incomplete 0 no no ae16.0
Its possible that packets are getting dropped
Please initiate a ping from MX to other router and capture a tcpdump on ae16.0 to figure out what's going on
monitor traffic interface ae16.0 no-resolve
monitor traffic interface ae16.0 no-resolve matching icmp
Its possible that either the device are misconfigured or packets are getting dropped in the middle.
Please share you configuration from juniper router and share a topology diagram.