Is not there. My carachter has done all BfA, and i’d use that portal hundred of times.
My recollection is that if you have talked to Zidormi and changed the zone’s timeline to before the end of Legion (Cenarion Hold instead of big sword*), the BfA portal won’t work to get you to Silithus (and maybe may not appear, I can’t remember).
Interstingly, if you did the Lunar Festival elder grind, you would have had to change the timeline phase to pre-BfA to get to a couple of elders, and it’s easy to forget to change back.
This is all on a character-by-character basis, too, which is why it might appear to affect only one toon.
*What sword? 
Extremely irritatingly, if you switch Uldum back (for example, to search for camel figurines) it also switches Silithus so you have to remember to undo the switch when you’re done or it will lock you out of the Silithus portal when you go back to Boralus.