At resolutions of 720x405 or lower, MTG Arena will, upon entry or exit of a game, force the game into windowed mode, and increase the resolution to 1920x1080.
Aside from this, while in full screen, any time the graphics menu is entered, the graphics will be reset to the highest possible graphics - in my case 3072x1920.
These two issues lead to a substantially reduced quality of my enjoyment as MTGA does not consistently run as well as I would like on higher graphic qualities, and the higher quality reduces color contrast, making cards difficult to parse with my colorblindness. This leads to me, any time I want to play a game, having to manually reduce the graphics quality before each game, which takes time, and is an extra thing to have to be worrying about during mulligans, because my game will occasionally crash at the higher resolutions.
As for the issue of going to the highest resolution any time you open the graphics menu, because the highest graphic quality is so high, I lose large parts of the outer edges of the screen at the highest resolution, my game has intense lag spikes that sometimes crash the client, and it makes it difficult to keep track of what graphical quality I want the game to be at.