Summary
Soul speed enchanted books can be found in stronghold library room chests.
Steps to reproduce
Use the /locate command to find a stronghold
Find a library inside the stronghold
Open the chests.
Repeat steps 1-3 in another world for several more times if you can't find a soul speed book (can't provide a consistent seed due to MCPE-72432)
The enchanted book will contain the soul speed enchantment.
Expected Results
Soul speed should not be obtainable through stronghold library chests
Observed Results
Soul speed can obtained through stronghold chests.
Screenshots/Videos Attached: Yes
Java edition also had this bug so it relates to MC-174690. It also relates to MCPE-71636. You can also obtain it through fishing. Soul speed is supposed to only be obtainable through bartering.