Reconnection is bugged since the SoI remastered update
When being disconnected from the server, reconnecting is not working correctly since the latest update. Regardless of whether you are actually temporarily offline, or not, or whether you are but you reconnect shortly afterwards, Arena is not correctly resuming your connection to the server as it did previously.
Example 1 is being disconnecting during opponents choice of who goes first. You check your connection or reconnect to the internet, and wait for Arena to notice. You can even hit 'Reconnect' when that button appears. It doesn't matter, you're not actually getting reconnected to the game. It will appear as if oppo is roping but you're disconnected. When you resume, you've probably been given some useless hand you would have mulliganned away, thus ruining the entire match.
Example 2 You're in sideboarding, there's no disconnection detection from Arena whatsoever. You can be literally disconnected from your router or pull the ethernet cable and Arena won't bat an eylid. That is of course until you try to hit 'Submit' on your deck, and nothing happens, but the clock keeps running down. So you reconnect to the internet in the 20 seconds you have left, and mash Submit, but nothing happens. Arena will not resume your connection to the game server during sideboarding. You are forced to take an un-side boarded deck into the next game.
Arena used to have a lot of reconnection woes, but since you added the ability to reconnect relatively 'seamlessly', without leaving the game, it has worked okay (although it does love to freeze up when you hit reconnect, forcing alt-f4). However since the Shadows over Innistrad Remastered update it is clearly no longer working properly.