Greetings! I have a couple of EX3400T running Junos 19.1R1.6 in a virtual chassis. An extended freatures license is installed on both. I have a 10Gb sfp+ installed connected to a MX480 router. The link between the two is up, passing traffic and is running clean with no errors. Problem is the link lights on the EX3400 do not light up. Link light on MX480 is up. Tried this with serveral different optics and even tried it with a 1Gb sfp. Same results: Link comes up, runs perfectly but no link light.
I have an ongoing JTAC case running on the exact same thing, just on EX2300 running 18.1R3-S3 (and -S4). Tried three different switches, both EX2300-24T and EX2300-24P. Same switches had working LEDs while running 15.1X53-D58 and -D59.
JTAC has confirmed that they can reproduce the behaviour in their lab and I'm now awaiting if there is a workaround or which release it will be fixed.
I would suspect that the same bug has hit your EX3400 running 19.1. Think you should create a JTAC case and refer to case number #2019-0423-0222. I have not received any PR number.
-- Best regards,
Jonas Hauge Jensen Systems Engineer, SEC DATACOM A/S (Denmark)