I've determined what the visual bug is. It's an artifact of the previous stage, the flame animation coming off of an enemy's ember axe. I'm not sure why it is the only animation I ever see do this though.
I saved right before sending the data since the game crashed last time, so no big loss. However, it may be useful for you to know that the visual glitch disappeared upon reloading.
Feedback ID with savefile:
0.3.7p_20230525-C8NLR