Yeah, it's because you guys need to have different height levels.
Within a multistory stair, Revit automatically "Groups" stairs of the same height, even if they are not sequential.
This are the Sub-groups I mentioned in my post.
Now, the first stair of the Sub-group, you can tab and reach the flights, while all other above, you cannot, unless you unlock the stair whose flights you wish to tag or select...
This is clearly visible in elevation or section, see attached sketch.
First TAB on bottom stair selects the sub-group (in this case 10' tall stairs)
The second TAB selects the stair, the third TAB selects the flight...
However, the flights of the top 10' stair can be tagged only if the stair is unlocked...
... and note that "tagged", "selected" and "tab-able" are interchangeable terms in this example.
I think there is a logic in grouping the same height stairs in a multistory, but I am not sure about preventing tagging the single flights in the stairs above above...
If we unlock and change the stairs, the tag should follow... right? It's a BIM software...?
Anyways, the workaround in this case is simple, just unlock the stairs above the first one to be able to tag its flights...
Thank you
Regards
gio