The mission Melting Pot is currently absent from the mission list. It should appear after Beyond The Nexus and before Scylla And Charybdis, but it is simply gone.
I've seen some people post about this on Reddit so I thought I'd add it here for visibility. At the very least this is affecting 25th Century and 23rd Century Fed characters.
Comments
However there will be a fix soon for it - they have found the issue (so it will be able to be played/replayed or skipped allowing continuing to the next quest that way, as I understood)
I already did it on my Jemm on the same day it appeared on available, thinking it could solve my own specific issue I had with a Jemm'Hadar (not related and not solved for me beside newly showing a non grayed out new frontiers tab - will have to complete mission six on Jemm's line 1st but that's beside the point. As for some others, doing the quest solved their specific New Frontier's line issue though, but my issue differed)
BUT maybe it's worths it to wait (I didn't understand if the quest itself changed or not though)