Mantoux test process. Test be incorrect?

My mother developed one boil on her neck six month ago. No other Symptoms. We consulted to Doctor, Doctor advised for Montoux test. The laboratory technician ( was fresher, without much experience ). He injected the liquid through the injection in hand DEEP INSIDE the SKIN and not on the top layer of skin. As result after three days the place where the injection was injected became red. Doctor (on the basis of mantoux test result ) advised for tb treatment. Though she was not having any symptoms. Before starting treatment lockdown started and we could not start the treatment for 3 months. After lockdown we went to doctor to start treatment, doctor advised for another mantoux test as it was already three month since the last test was conducted. We got tested in another lab. This lab technician injected the injection in the outermost layer of skin. After three days, it came Negative. Doctor also agreed to not start the treatment as the report is negative. Can mantoux test be incorrect?