Visual Studio 2019 Preview 2.2 includes an updated blue theme that makes inactive tab text in Tabs Studio invisible:
One workaround to fix it is applying the following Tabs style:
<Style TargetType="TabsStudio:Tab" BasedOn="{StaticResource DefaultTabStyle}">
<Style.Triggers>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsGroupFocused" Value="False"/>
<Condition Property="IsTabSelected" Value="True"/>
</MultiTrigger.Conditions>
<Setter Property="Foreground" Value="{DynamicResource {x:Static vs_shell:EnvironmentColors.FileTabInactiveTextBrushKey}}"/>
</MultiTrigger>
</Style.Triggers>
</Style>
The fix will be included in the next Tabs Studio release.
Tabs Studio Blog (organizing Visual Studio document tabs) 