Summary

Same issue in MC-188693

The FOV stays the same when a player starts riding a mob after standing on Soul Sand with boots have Soul Speed enchantment. FOV would revert normally because you are no longer staying on Soul Sand.

Addons/Resource Packs: No

Attachments: Yes

Steps to reproduce

1. Take a boots and enchant with soul speed
2. Turn on FOV altered in settings
3. Stand on soul sand 
4. Spawn a pig or a strider with a saddle off the soul sand
5. Click the riding mobs

Observed Results

The FOV altered leaves remains higher after the player ridding some mobs that leave from soul speed 

Expected Behavior

when a player ridding to mobs, The FOV will not staying higher after leaving from soul soil