Why the sudden Strength drop?

I rested 2 days and the night before my workout I had enough sleep and I had an adequate amount of carbs at least 2 hours before my session. Now when it came to doing it I was very weak, I could barely do 2 reps with my weighted chin ups, I used the same weight that I could do 6 reps last time with, but it was a grinder. Also I've gone into a deficit for a week and lost 1kg, but I have been spot on with my macros.

Any reason for this drop in strength? Any tips to prevent it?
Why the sudden Strength drop?
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