MainStash doesn't really guarantee permanence. This will get a lot worse after T383327: Re-architecture mainstash (x2) to allow easier maintenance .

As long as it's not setting TTL to infinity, it actually should get better. Previous main stash infra was redis that was actively evicting keys. This one puts it on two db hosts and they will be lost if both hosts get depooled at the same time which is much lower chance of happening than x2 having issues or redis evicting keys.