Some NBT data associated with a wandering trader (summoned by commands) is completely reverted when logging out of the game and coming back. The reverting of the NBT also seems to be irreversible. The biggest example of this is the Offers NBT, where if you set a trader to not have any offers, relogging will suddenly make normal offers appear. This cannot be changed, even with /data commands. On another note, /data in general seems to be very odd with the trader after relogging. /data modify, for example, does not appear to work at all.