Thanks to the folks that reached out. We think we understand what happened here - combination of some things shipping in the Steam beta for the Deck, plus a crash bug that looks to have affected a small group of users. We know being unexpectedly logged out sucks, so we take issues like this seriously.

Fix should be included in the next stable Steam client release.

In the meantime, for those of you using Offline mode ahead of travel - you may want to experiment with just using airplane mode instead.