The only reason that would happen is them or their pet casting Steal Ward if you are talking about an opponent taking the shield. If you mean that it's casting on the wrong player in your party, then it's you. Use the name box at the bottom of the screen to target casting, not the wizard.
Are you sure you're not accidentally clicking another wizard in battle? I think it's something we've all done at some point and mis-clicked our target.
One thing that can help with that is turning on the 2D Combat Display under "Gameplay Options" in the Options menu. You can click the name of the target to target your spells rather than the actual avatar themselves. It can help prevent that mistake. Try that and see if it keeps happening.
i hope you know that when i was fighting a balance boss with no steal ward on a boss with my friends i used a tower shield and it went on her pls think
This was answered. It has happened to everyone, it's not the spell or a true bug. Use the 2d combat display to choose targets to make sure you are choosing the correct person. This can also happen if someone flees after you chose the target. It will cast on the next person in line throwing off blades and shields that were cast during that round.