Your follow list is stored somewhere. Some clients store it on relays, some on your device, or both.

When you try a new client it doesn't know about your existing follow list, so it needs to fetch it from somewhere, but where? It doesn't know which relays you used in the other client. So first you need to fetch your list of relays, but this is the same problem, where do you fetch the relays from? So clients just have a list of popular relays hoping they can find your relay or follow list from there.

How can your follow list be suddenly wiped? The follow list is a list that is replaced on relays when updated. When you follow someone new your new follow list will be 'your existing follow list + new follow'

This means if there is a problem with fetching your existing list, your new follow list will be 'empty + new follow', aka wiped.

It could also be a problem with different clients not being able to correctly read your follow list. Originally follow lists were just lists of contacts, more recently clients have started putting hashtags in follow lists, potentially causing other clients to fail in unpredictable ways.

This problem will never be solved until clients start storing local copies, in addition to publishing to relays.

So if there ever is a mismatch, you can always go back to your original client to restore.