Having this same issue on my HP laptop running Linux Mint 19.3 MATE. I only play Stardew Valley and occasionally remote-play Elite Dangerous on this machine as I have a dedicated Windows 10 gaming desktop PC downstairs (from which I often stream Elite).
Twice now I've gotten a 'low disk space warning' error on my laptop to find out it is the /tmp/dumps/user_stdout.txt file. I have a 500GB hard drive and about 240GB of it is free, but when I checked the aforementioned user_stdout.txt file it was taking up 200GB of space and killing my hard drive!!!
Nothing extracted yet.
Compatibility Report
System Information
I confirm:
Proton log here --> http://ulujain.org/steam-704860.log (500Kb file)
Symptoms
Game generates nearly 15Mb of logging data in /tmp/dumps/user_stdout.txt in about 1 minute of game time. Yes, I know this file is cleared on reboot, but it s woefully excessive.
Before and after ls -al of the directory:
https://imgur.com/Imq9Xs0.png
Reproduction
By playing the game. Happens every time, windowed or fullscreen. The game itself runs flawlessly, music, sound, movement, saving, loading, graphics etc, are all good.