Acute on chronic liver failure(ACLF) is a life-threatening disease, a transplant may be needed, I hope your doctor got the diagnosis wrong or used the wrong term.
In chronic Hepatitis B, it is not unusual to have a high ALT(sgpt) flare. It could be due to several reasons such as flare in the Immune Clearance phase, flare due to drug resistance developed during oral antiviral treatment, or flare due to immune escape in the HBeAg negative chronic phase.
Any flare in SGPT(ALT) if you have cirrhosis is dangerous.
Just my opinion.