Micro-BFD (Bidirectional Forwarding Detection) is a method of running individual BFD sessions over each member link within a Link Aggregation Group (LAG) to monitor their bidirectional liveness and rapidly detect failures. Unlike a single BFD session over the entire LAG, Micro-BFD provides granular detection for each underlying link, ensuring that a link is considered operational only when its associated Micro-BFD session is established and active, leading to faster network reconvergence in case of a member link failure.

If a micro-bfd session is configured to use the IPv6 interface address rather than the IPv4 interface address or the loopback interface, the bfd session does not get established

The MicroBFD session remains up if its configured between 2 ACX devices but fails if its configured between ACX to MX or MX to MX.MicroBFD sessions remain in the down state on child members of AE interface. when an IPv6 interface address is used Duplicate Address Detection (DAD) is active on the AE interface. DAD prevents the interface from being considered fully operational until it confirms the uniqueness of the IPv6 address. Since MicroBFD packets are not specially exempted from DAD checks, they are dropped or ignored during this phase. 

labroot@mx204-1# show interfaces ae1
aggregated-ether-options {
bfd-liveness-detection {
minimum-interval 1000;
neighbor 2001:16::2;
local-address 2001:16::3;
}
link-speed mixed;
lacp {
active;
periodic fast;
}
}
unit 0 {
family inet {
address 172.16.0.3/31;
}
family inet6 {
address 2001:16::3/127;
}
}

labroot@mx204-1# run show lacp interfaces
 Aggregated interface: ae1
     LACP state:       Role   Exp   Def  Dist  Col  Syn  Aggr  Timeout  Activity
       et-0/0/2       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
       et-0/0/2     Partner    No    No   Yes  Yes  Yes   Yes     Fast    Active
       et-0/0/3       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
       et-0/0/3     Partner    No    No   Yes  Yes  Yes   Yes     Fast    Active
     LACP protocol:        Receive State  Transmit State          Mux State
       et-0/0/2                  Current   Fast periodic Collecting distributing
       et-0/0/3                  Current   Fast periodic Collecting distributing
 
 
 ===BFD state===
 labroot@mx204-1# run show lacp bfd-interfaces ae1
 Aggregated interface: ae1
     LACP state:       Role   Exp   Def  Dist  Col  Syn  Aggr  Timeout  Activity
       et-0/0/2       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
       et-0/0/2     Partner    No    No   Yes  Yes  Yes   Yes     Fast    Active
       et-0/0/3       Actor    No    No   Yes  Yes  Yes   Yes     Fast    Active
       et-0/0/3     Partner    No    No   Yes  Yes  Yes   Yes     Fast    Active
     LACP protocol:        Receive State  Transmit State          Mux State
       et-0/0/2                  Current   Fast periodic Collecting distributing
       et-0/0/3                  Current   Fast periodic Collecting distributing
     BFD protocol:         BFD State
       et-0/0/2             Initializing
       et-0/0/3             Initializing
 
 [edit]
 labroot@mx204-1# run show bfd session
                                                   Detect   Transmit
 Address                  State     Interface      Time     Interval  Multiplier
 2001:16::3               Down      et-0/0/3       0.000     1.000        3
 2001:16::3               Down      et-0/0/2       0.000     1.000        3

As a workaround 
Disable dad process on AE interface
set interfaces <xyz> family inet6 dad-disable