Thanks for your answer, but I probably didn't well explained the issue.
I have an external TabGroup; inside the first tab there is an internal TabGroup; inside the second tab there is another internal TabGroup.
By default, the first tab of the external TabGroup is selected, along with the first tab of the internal TabGroup of the first tab of the external TabGroup.
If I click on the second tab of the external TabGroup, the default selection of its internal TabGroup is its first tab, but it appears as if no tab is selected at all.
Only if I click on purpose on the first tab it appears as selected.
I can confirm the first tab is selected by default even if it doesn't appear so because what it's shown is the content of the first tab, instead of the content of the second tab. The issue is only graphical.
The issue is reproducible with any internal TabGroup inside a tab of the external TabGroup except for the first one.