TL;DR: I exercise regularly and want to lose some weight. I am doing a calorie deficit diet on days that I don't exercise. Would I lose muscle mass?
Some background: I am 171 cm, 71 kg, 33yo and male. I am not particularly athletic, I can finish 5k in thirty minutes or 10k with a lower speed, but that is all. My stomach is a bit protruded so even if I am very skinny, it looks as if I have some beer belly. This beer belly doesn't look bad if I am 66-67 kg, except I have a large dinner (how much I ate is evident to everyone from my belly).
I exercise regularly, 2-3 times a week. My exercise is comprised of 12 moves (push up, plank, crunch, sit up, russian twists, back hyper extension, leg raise, dumbbell lateral raise, dumbbell standing triceps extension, sit squad, dumbbell shoulder press, dumbbell biceps curl). They weren't prepared by an instructor so they might not be the best set of exercises. I just chose these moves to improve my core and upper body strength. I have already quite thick and reasonably muscular legs, probably because I use bicycle to commute, so that no one would ever tell me if I skipped leg days.
I am currently trying to do a calorie deficit diet with 14h fasting. Using Harris-Benedict Calculator, I found that my basal metabolism is 1670 calories/day. I am not very active due to the lockdown so I think the multiplier 1.2-1.3 fits for me, except the days I am exercising. I count my protein intake as well, on my exercise days I make sure to have 100 grams of protein, from various sources, (eggs, cheese, meat, canned tuna, nuts, Huel). I take in around 1600-1700 on days I don't exercise and 1900-2100 on days that I do exercise. In other words, I don't think I am calorie deficit on the days that I exercise.
My long term goals are to stay in shape, reach to a weight of 66-67 kg and build my core and upper body, as well as to fight depression due to the lockdown blues.
Question: If I keep having enough protein but use a slightly calorie deficient diet on days that I don't exercise, should I expect to lose muscle mass?