Aurora 4x

Proton 5.0.6, the game is Aurora 4x
OS is Linux Mint 19

I've added the executable to steam as a non steam game.
In steam properties for the game, I've 'Force use of Proton' 5.0.6.
Click Play, a ui box flicks up and immediately disappears.
Added PROTON_LOG=1 %command% to the launch options.
Click Play, check home folder, no new logs have been created.
I've checked that the executable has permissions to 'allow executing file as a program'
I've checked the path, everything looks good there.
I can run the game natively with some mono hacks (so I know it does run), which is my preferred way of running the game, however there's a few bugs which are forcing me to find an alternative.
I've copied the path from the steam properties, and run it from the command line, so I know the path is correct.

It appears that steam is failing even before proton gets a chance to create a log. it looks like steam is popping up an error dialog and immediately closing it.

I've even tested with another proton game to ensure that I'm looking for the log in the right place (I am)

No source files, tests, or code entry points are named. Start by reproducing Aurora 4x from Steam with Proton 5.0.6 and checking whether PROTON_LOG=1 %command% creates a log; done would require identifying why launch exits before logging and documenting a confirmed fix or diagnostic.