Post covid lung Fibrosis. Spo² levels?

My father  got fever and tested covid positive on 06-May-21, Phase 1: With the advice of Relative Doctor Home Isolated and started medication from Day 1 Till Day 12 his health is sound and stable Phase 2 From Day 13-17 Oxygen levels are decreased from 98 to 81. And started Steroids IV 1 Inj.Piptaz 4.5 g IV TDS in 100 ml NS 2 Inj. Dexamethosone  4 mg IV BD Phase 3 From Day 17 Oxygen support started initially with 4 Ltrs with Spo2 of 96 Gradually Spo2 levels are increased hence we have decreased gradually O² supply to 1 liter, till yesterday SPO2 was 96. Phase 4 Yesterday All of sudden  Spo2 levels on 1 liter O² support dropped from 96 to 91 then we have increased O2 support to 3 Ltrs and spo2 is now 96 on 3 ltr. (Later we have noticed that the O² pressure was at Zero for two hours, is it the reason for Sudden dropping of SPO² levels?) He is recoverd from covid and yesterday we took CT scan and found 30-40℅ lung involvement. - reports and medication attached