While walking or sprinting up carpeted stairs the player collides with the carpet block causing the player to start going prone as you would using a piston or trapdoor. It doesn't seem to cause the player to ever go fully prone but rather cause a "stuttering" effect. I tested it using only Inspirations and Mantle.

• Minecraft: 1.16.3
• Mantle: 1.16.4-1.16.43
• Inspirations: 1.2.1.24

Fixed in 1.2.2