Width of sections/columns in Mail view not saved; changes when I relaunch the app.

The leftmost column in the Mail view (that shows email accounts and folders) sometimes opens so narrow that I can’t even read much of the text. The column showing emails seems to open either narrower or wider than I set it last time I had the app open. I’ve resized these many times, but when I totally close and relaunch the app, those settings don’t seem to stay. 

I should note that I do go between using an external monitor (1920x1080) and the screen on my Surface Book 2 (3000x2000). But this problem seems to happen even when I haven’t switched monitors between closing and relaunching the app.