From Wickd's Twitter: https://twitter.com/Wickdlol/status/1377337267866374146
It's in the title really, the new Trinity Force has to be in an item slot higher than your boots for the movement speed bonus to apply. In the responses to the tweet above you can clearly see it in action.
With the buffs to Trinity Force in this latest patch I felt like this was something people should be made aware of, so that they at least get the full value out of their items.
EDIT: To clarify, this only applies to the bonus movement speed from the Mythic Passive, i.e. the bonus movement speed you get per Legendary item.
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