ChatGPT summary of the disappearing follow list problem and the status of resolution. I suspect the TL;DR is - back it all up periodically. It's going to keep happening until every client I use supports NIP-65:

Nostr users often experience their follow lists disappearing due to a combination of factors: local storage loss, client bugs, missing relay synchronization, or switching between clients that don’t re-import historical data. This is because follow lists are stored in Kind 3 events that can be lost if relays prune them or clients fail to re-fetch them properly. To address this, NIP-65 introduces Kind 10002 “Relay List Metadata” events, which let users publish a signed list of their read/write relays. This helps clients reliably locate where a user posts and reads events, improving follow list stability and relay discovery. While the NIP-65 spec is finalized and some clients already support it, adoption is still growing, and many users remain reliant on older, less reliable methods. Broader support and regular publishing of Kind 10002 events are key to solving this problem at scale.