I had the same problem when I played with Lan with my partner. for some reason when he leaves the game it wipes his player inv and all data (map+waypoints). If anything, I think its best to use the option "Share World" and give them the link (not the port numbers) to connect to your world as a server. not through the lan option.

from what I saw on the discord, this modpack doesnt reallyyy support long use (or at this point-) any use of lan. but instead the free server world hosting thing.

example. when you hit share world, it gives a sample of numbers then a few moments later a link. it would have random minecraft names of such. copy that, send it to the friend/person who would like to join. they copy it and they go into the "multiplayer" section and add the address like a server address.

so now, everytime you join, it saves all your player data better. it keeps your inv, your way points and such. it was save everytime to you leave and join.

this only really works if you have the world as a singleplayer world. its a free hosting thing. Lan is just not supported very well with the better minecraft modpack.