1st. Did you use the in-game random map designer, or Galaxy Forge for v1.15?
2nd. The likely reason the cannot join your game is that they do not have the proper map files. Unless you have uploaded/distributed your map, or some such thing like that.
Overall, the problem you describe mostly has to do with the fact that, since you have manufactured your maps correctly, and you can host a game with said maps, it simply is that the likely do not have the map.
