It looks like the problem is with the --no-sandbox flag that is passed by $SNAP/command-slack.wrapper. Manually running the command without that flag fixes the issue, but that’s not helpful for regular users that aren’t going to want to be messing with commandline hacking 
@advocacy this might be something for you to follow-up on?
Specifically, from snap run --shell slack, this works:
$SNAP/usr/lib/slack/slack
While this crashes:
$SNAP/usr/lib/slack/slack --no-sandbox