Additional (Hopefully the final) update:
After significant trial and error, I have discovered that the problem is not a bug per se that I know of, but more a problem with too many resource-heavy custom islands being on the cartography map at the same time . When I was having these problems, I had all 20 islands replaced with custom islands ( like I said, I’m not very good at this game yet).
The workaround that I found previously, allowed for resources to spawn back on the island upon saving and reloading, however this would make my newly created shelter on said island, as well as my raft disappear. I believe this was a problem other people were having as well.
Eventually I discovered that as long as I kept the amount of resource-heavy custom islands to 8 or less on the cartography map, the problem of resources despawning on a map when you approach, as well as the problem of losing your shelter and raft when reloading a save, both stopped happening.
The only reason for a problem like this that I can think of, would be too many resources overall throughout the entirety of the cartography map. With custom islands, you are able to make some resource-heavy islands if you wish, to make the game easier. However, too many of them on a map might be exceeding a possible maximum amount of resources that the game is capable of handling at once.
So when you approach a new island on an overloaded map, the resources would disappear. Upon saving and reloading on this new island, the resources would be back but your shelter and your raft and potentially any additional structures you had built there would disappear. Not entirely certain if this makes sense, but it’s the only thing I can think of off hand. LOL
I no longer have any problems keeping my structures and raft, and I also no longer have any problem with my islands losing everything I put on them before I even get there. Hopefully this information can prove helpful to other players, who may also be having similar problems with things disappearing on their map. Currently, I have no new issues to report, but if I do, I will make sure to post them as well as any potential workarounds that I discover.