A 3x2 unit is being left flanked by 1x3 leonx touching only the 2º rank. In that position is able the first unit to reform such way it wont be flanked?

It seems Ilegal movment.

Acording reform rules you can pivot on your center point or center tray, and both cases dont deny the flanking situation.

That's my understanding. If you reform around the center, you won't be engaged. If you try reform around the engaged tray, your front rank would overlap the Leonx Riders, and would therefore be illegal. As you hinted at, there are only 2 legal reforms: 1) a 180-degree turn around the center of the unit will make the Leonx engaged with the unit's front-left tray, still flanking; 2) a 90-degree turn around the tray that is touching can result in the back edge touching instead of the side, but the Leonx would still be flanking.

Thanks Budgernaut. Im going with that understanding.

If you're willing to take 2 reforms to get to it, you could first do the 180 reform, THEN reform around the contacted tray. But I can't imagine many situations where taking two turns to become not-flanked is the right move.