Mirza Ghulam Ahmad was intoxicated during Salaat. He was fond chewing paan, which is a huge problem in the indian subcontinent. Further, it is against Islam to approach prayers while intoxicated, there is a famous verse in the quran that MGA violated.
“Dr Mir Ismail narrated to me that once hz saheb (mgaq) had a severe cough attack, so much so that he would lose breath. But keeping a pan (beetle leaf) in mouth would give him much relief. At that time, in that condition he prayed having pan leaf in mouth so that he could pray with ease.”