There is a big, long explanation for this sort of problem in direct sail...

First, some of the "island locations" are a single island model, and some are the main location model, with several distant islands visible in the distance, but are technically not really valid locations, but just for scenery on the horizon.  In original COAS, if you sailed directly to those distance islands, you could reach them, and even keep sailing beyond them.  But if you reached that island and sailed around the circumference of it, you would never actually see the ports, or any detail, even though it was defined as a real location, until you exit sea, enter world map, then reload back to sea.  Then, you would then get the valid location to load of that once distant island to load it's real model, not the fake distant one you saw from afar.  For those islands that are just a single model, with no distance islands visible, you can sail forever in a direction where you know another location exists, and you will never reach it in COAS, because there is no such thing as "direct sail" ability in the original game.

The modders for GOF added the "direct sail" feature to calculate the distance you sailed from your original location, compare it to the distance from every other island, and if at some point during your sailing, if the distance from any other worldmap location was less than the distance you had sailed from your original location, their modded scripts would force that location's model to load and you would get a loading screen as you transition from that portion of the map to the other.  This usually works except for transitions from certain locations on the western and southern parts of the map.

The reason those western and southern parts of the map are a problem, is that it turns out the original design of COAS is that the worldmap for Cuba and the South American continent on the worldmap, and their respective port locations are actually a different distance scale than all the other portions of the map, so comparing the distances doesn't work well, because if you started sailing at any of the eastern island, then approached Cuba or South America (both west and south on the world map), at some point in the journey, the distance scaling prevents the calculation, because there is no way to account for when the distance scaling needs to change, because that dividing line of distance scaling change is unknown.  In GOF, if you sailed from Hispaniola, toward Cuba, you could reach Cuba shores and nothing would happen, unless you turned north for a ways and eventually you could get the transition.

For ERAS, which is based on the GOF mod, a change was made to help alleviate this problem by using the direction your ship is heading at any given time, and if any of the "problem" ports are forward of your position, it will use a different scaling for calculating your distance to those ports, and the regular distance calculation for ports not on that last of problems, to help eliminate the problem.  It's still imperfect, but it works better for most locations that GOF did.  I have tested that sail from Hispaniola to Cuba in GOF, and you can reach the island without a load, but in ERAS, you get fairly close and can see the low-res fake island, but the transition happens sooner than GOF, before you reach the shore.

This has been a known issue since the early days of COAS and GOF and also a problem in the New Horizons mod when they lifted the South American portion of the worldmap from COAS and put it into POTC.  There are discussions on PiratesAhoy about this very issue, and the attempts to compensate for the scaling differences in the modded scripts.