What I expected to happen was...:
I expected soul speed and depth strider or frost walker to be mutually exclusive, meaning they can't combine in an anvil.

What actually happened was...:
Soul Speed combined with depth strider and frost walker. 

Steps to Reproduce:
1. Obtain a soul speed enchantment book from bartering or creative.
2. Enchant boots with soul speed. 
3. Obtain depth strider or frost walker, and combine it with the boots with soul speed. 

Boom. They combine.