Champ dissapearing and not allowing replacement

Okay so basically I was playing, had a lvl 1 sejuani on the field and one on the bench, I then then leveled up to 7 during combat(after sej died), then when combat ended she wasnt on the field and i wasnt allowed to replace her, when i bought my 3rd sej no1 upgraded. Its like sej wasnt in my team for a whole round since after the next round ended she reappeared and upgraded to 2 star.