How quickly do you lose muscle in a cut?

Hey all, hoping you can shed some light on my weight loss.

I’ve always been a skinny boy. I’ve spent the last two years or so putting on weight and muscle in the gym (about 15-20kg altogether).

I recently decided to start losing weight - I want to go from 87kg to 80kg by the end of the year. I’m using MacroFactor and sticking to my PPL routine.

However, almost immediately I started struggling to meet the same benchmarks I set for myself at the gym. How quick does this process happen? It feels like I’m losing strength so quickly, and every day I lose a rep or two for each set.

Am I freaking out over nothing?