@esssbeee This is a typical side effect of MAC and bronchiectasis. There is no single cause everyone can agree on, but it may be that the infection itself is a) causing your body to burn extra calories trying to heal or b) causing malabsorption of nutrients.
Either way, I understand your concern. I have the same experience, as well as being on the Big 3 antibiotics for over a year. I have lost weight to the point that I nearly look anorexic (15% loss from my previous weight), and I'm not happy about it.
Here are a few of my strategies, which have allowed me to maintain (but not regain) weight for the past couple months
Probiotic to help gut (due to antibiotics)
Force-feeding myself - an high protein or high calorie snack every 1-2 hours whether or not I'm hungry - 1 oz cheese or meat cubes, 1/2 of a protein drink, 1/2-whole protein/snack bar, banana or apple, yogurt, even ice cream. I stop after dinner due to needing to take my meds on an empty stomach.
Full-fat dairy products (I don't have a cholesterol issue)
Carnation Instant Breakfast in my morning coffee
Smoothie - yogurt, multiple fruits, protein-vegetable powder, carrot & avocado chunks, fruit or veg juice - I make 2 liters at a time, store in frig, keeps about 4 days. I can manage to drink this even when my stomach refuses solid food.
Ditch salads because they fill me up too much, when I do eat them I load with meat, cheese, eggs & full-fat dressing.
Add butter or sauce to all veggies and potatoes.
When you go to NJH, ask for a consult with a nutritionist, especially if they start you on antibiotics. I wish I had that option at my clinic when I started.