Odd, never seen that issue before, you're supposed to get a completion stats screen after the boss dies. I'll have to investigate, thanks for the report
I was able to load back into the save and after defeating the boss it did play, so not sure what happened. Another bug: on a second playthrough I went to pick up all of things I missed and after picking up the pulse cannon, I could not switch from it no matter how I rebound the weapon switch keys. Oddly enough, even after picking up the shock gun it did not switch to that weapon automatically but instead left me locked on the pulse cannon; the only time I was able to switch back to the pistol was by dropping the combat rifle and on one instance it sent me back to the pistol instead of the pulse cannon. So bug fix needed here. That being said this is probably the most interesting game I've seen in a long time; I love 2D freeaim shooters and the atmosphere feels truly terminally abandoned, as if it this facility has been so long in ruin that upon awakening the character's existence feels displaced to what is actually there now.