The flatpak version of steam does not open on awesomewm but will open on gnome and kde. This issue started happening when i reinstalled endevouros to boot for uefi systems (i wanted to make a windows vm). Here is the pastebin of the logs https://pastebin.com/WqAAQN4m
welcome here at the purple side 
i do not call myself an expert on flatpaks… the only “fatal” error the log shows is this.
src/steamUI/steamuisharedjscontroller.cpp (540) : Fatal assert; application exiting
could be the journal from the same time could show hint…in addition as it is only happen in awsome-wm … it could be something outside steam itself and related to the WM.
you can check journal while running this in a terminal:
journalctl -f
or create log file by using timespan option:
journalctl --since "10 minutes ago"
you can write this to a file to review (on your own … never post journal without checking it for personal data it can include)
journalctl --since "10 minutes ago" > ~/10-min-journal.log