The Enchancement mod adds an enchantment, Slide, which can put the player in a crawling state. First Person Model doesn't hide the playermodel when the player is crawling due to Slide, so when using the enchantment the player's view will be obscured.

I have recorded a short video to demonstrate the issue:

I have also reported this on the Enchancement repo. The corresponding issue report there is MoriyaShiine/enchancement#51

During crawling the player is not hidden, it's behind the camera. You still can see the arms while crawling. The Slide enchantment doesn't seem to offset the player model at all, so the camera is in the middle of the torso instead. Could probably be fixed from either side.