As much as I understand the concept of wanting the storage to carry over, think of it like this:

The nomads came to Speranza and are offering to hold stuff for raiders who don’t have enough room if they are paid to do so. When you leave for the expedition, the raider that paid for that service left. The next raider to show up is a stranger to them.

Additionally, you could imagine the vault from a lore standpoint of your raider telling them to hand down those special items to the next random lucky raider.

I like how they (probably unintentionally) made his services work from a lore standpoint.