The Truststone equipped to my Trust Master equipment changed without my doing anything.

When you add a unit to a formation that has a Trust Master equipment item registered as a favorite, the favorite settings for slotted Truststones will also be applied when the equipment is automatically equipped.

■ Example
Equipment Setup:
Lion Emblem with a Truststone of Idealism (Vital) and a Truststone of Justice (Vital) set to it.

Favorite equipment for Mont Leonis:
The Lion Emblem is selected as a favorite. The Truststone of Idealism (Shield) and Truststone of Justice (Shield) are selected under Equip Truststones to Favorites.

Favorite equipment for Sterne Leonis:
Lion Emblem is selected as a favorite. No Truststones are selected under Equip Truststones to Favorites.

With the above equipment setup...
If Mont Leonis is added to the party, his favorited Lion Emblem will be automatically equipped, with the Truststone of Idealism (Shield) and Truststone of Justice (Shield) set to it.
If Sterne Leonis is added to the party, his favorited Lion Emblem will be automatically equipped, and all the Lion Emblem Truststones will be removed.