When you are at the Endgame, the Safe House Salthamm on the Archipelago doesn’t show on your Map so you cannot fast Travel at this location even if you go yourself in to re-unlock it. This bug is appeared since the February update.
Are you sure there aren’t more that are missing on your map? That’s what happens to me for about a year now.
I’m not sure, but once someone said that it’s a know and famous bug on Playstation.
100% sure! I have already unlocked every single Safe House (Thanks to the Collectibles) and Salthamm is (On my Ps4! Don’t know by the Others.) the only Safe House that has still the same issue like the other Safe Houses previously!