It happens on any tier of boots enchanted with Soul Speed I or higher. When a player in creative mode wears these boots when walking on Soul Sand our Soul Soil, they decrease durability because the enchantment applies. Tools and armour do usually not decrease durability upon use in creative.

To reproduce:

1. Take some boots with soul speed enchantment
2. Walk around a while on some Soul Sand or Soul Soil in Creative mode
3. See the boots get durability decrease:
   

See SoulSpeedBug480K.mp4 for a video.