This issue only happens when equipping Procipinee's Crown. It is unique because it is an accessory that has 3d graphics.

When equipping it while making a custom sovereign, or equipping it using the inventory system for champions, the 3d model will appear in the 3d representation of the model. However, as soon as the custom sov is made, or the inventory system is closed, the 3d graphics will disappear from the model.

Here is an example of a unit that I equipped the crown on. The top part of the picture is from custom sov creation, and the bottom part is how it looked on the sovereign selection screen (and later in-game).

Edit: Sorry, unmodded game: