If any tabs are undocked from the main viewport and another tab is then docked to the main viewport it will hide any undocked tabs. This only appears to affect tabs that in the Menu > Window with exception of Project and Editor settings.
See attached image gif for demonstration
2. Undock any of the four main tabs (Details, Modes, Outliner, Content Browser)
3. Open a static mesh or blueprint
4. doc the opened tab to the main viewport if it doesn't do so automatically
5. undocked tab disappears
Head over to the existingAnswerHub thread and let us know what's up.