I am recently hitting an interesting problem with MXs on HA pairs.
Procedure to replicatie problem:
Setup a HA pair with static IPs for the uplinks. Test everything to be working ok.
Reboot or remove power from one of the MX devices. The device may never join the dashboard.
Further troubleshooting (pcap from a switch), shows that the problematic MX has booted with an uplink IP of 1.1.1.1 and is looking for 1.1.1.2 .
Only way to solve this, seems to be factory resetting the MX and reconfiguring it, or suppling a DHCP on it's uplink.
I've seen this on MX68 & 84 on 14.53.
I have an open case with support on this, but I was wondering if someone else has seen this behaviour.