Re: QFX5120-48Y VC-port with different speed

The main problem has probably been that virtual chassis isn't supported on QFX5120-48Y before Junos 19.3R1. Using 18.4 with VC would have given strange issues like the one described.

Ref. https://www.juniper.net/documentation/en_US/junos/topics/concept/virtual-chassis-qfx-series-understanding.html

I would go with Junos 19.4R2-S2 for a start if VC was needed on QFX5120-48Y's