The bug

Custom text colors defined for an advancement title are overwritten by the advancement type default one in the hover text.

@dinnerbone: MC-117318: cough cough "and therefore also keep custom formatting"

How to reproduce

1. Download the attached advancement title_test.json and place it in the following directory of your world folder

   data\advancements\custom
   

2. Load the world
3. Grant yourself the advancement

   /advancement grant @s only custom:title_test
   

4. Open the chat and hover over the advancement title