How to reproduce:
1. Create a new Unity project
2. Make sure to use the default Editor layour
3. Navigate to the Scene/Game view tab group
4. Drag any of the tabs and observe the results

Expected results: The text on the tab header doesn't disappear during the drag
Actual results: The text shortly disappears 

Reproducible with: 6000.3.0a3, 6000.3.0a6
Not reproducible with: 6000.0.56f1, 6000.1.16f1, 6000.2.1f1, 6000.3.0a2

Reproducible on: macOS Sequoia 15.5 (M1)
Not reproducible on: No other environment tested

UPDATE:

In addition to the text briefly disappearing, the Alpha of the tabs is incorrect: they become semi-transparent while still in the dock. Tabs should remain opaque within the dock and only become semi-transparent (on macOS) when extracted.