Ah
NOW I see what you mean!?
Sorry for the misinterpretation.
The undo action takes the focus away from the entry that was inserted during the erroneous declaration action.
This lost focus turns it to a "not allowed to declare as known" state!
Detail description:
On PocketStackz, after an erroneous declaration, the undo action adds the previously declared entry ("A") back to the local stack, without removing the entry that replaced it during the erroneous declaration ("B"). After the undo action, "A" is selected in the list and its attributes are displayed. When the new entry "B" then is selected, the "declare as known" button is disabled for this entry because it is selected for the second time now, and "declare as known" is only permitted at the first selection to avoid positive declarations out of an iterative learning session.
We will try to find a solution to this problem.
In the meantime, the entries that are added before an undo action can't be declared as known in a learning session indeed. They are best put back on the same stack instead, which does not alter their statistics. Note that another session can be started directly afterwards to catch up on classifying the missed entries, a test session would be more appropriate than a learn session in this case.