How to reproduce:
1. Open the attached “IN-105576“ project
2. Open the “Main” scene
3. In the Project window, go to the UIAssets folder and open the “Underline.uxml” file
4. In the open UI Builder window, in the Hierarchy window, select “#TestThisOne“
5. In the Inspector window, go to the Text tab and apply the bold effect by clicking the “B” button in the Font Style
6. Observe the underlined text in the Viewport window

Expected result: The bold effect is applied to the text, and the whole text is underlined
Actual result: The bold effect is applied to the text, but the underline partially disappears

Reproducible with: 6000.0.0b15, 6000.0.51f1, 6000.1.9f1, 6000.2.0b7, 6000.3.0a1
Not reproducible with: 2022.3.63f1, 6000.0.0b14

Reproducible on: MacOS 15.5 (M4)
Not reproducible on: No other environments tested

Note: The issue was reproduced using the “ABCSocial-Regular SDF (Font Asset)“