Make sure that you're not running out of stamina and getting a Game Over. If you do, the game won't have a chance to save the scene.
Make absolutely sure that you see the scene all the way through and then defeat the slime afterwards! Make sure you're all the way back to regular exploring gameplay. If you've got the slots for it, make a new save after it's done, BEFORE going back to your gallery save.
If it's still not unlocking for you and you can confirm you've done these things, we'll look into it and see if there's any issue with it. This one is a liiittle bit finnicky but it shouldn't be bugged according to the others who've gone through it!