Are you able to reliably reproduce the soft lock? Tips for how to do it would be appreciated - thanks.
Okay here's what I do:
(Using black N3DS, latest FW 11.4 using Luma3DS 7.0.5 stable)
Installed the homebrew through CIA using FBI's URL CIA installer.
Only two files; doom.wad and doom2.wad, are inside /3ds/prboom3ds/.
Started new game in fresh file, no changes in any game config.
Knee Deep as the episode of course, in Ultra Violence.
Go through E1M1 through the main path, grab armor, kill all baddies on the way without unlocking any secrets.
Once I get to E1M2, all it takes is just a couple of shots to the two guys at the hallway right in front of where you start and it locks up there.
It sometimes went a bit further, but letting them kill me in E1M2 and respawning in the level, then shooting the two guys again crashes the game as well.
Let me know if you need any more help with this, if you still can't reproduce it, I might take a video of it to be more clear.