@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.

@esssbeee I moved your message and question about how to stabilize weight with MAC to this existing message about weight loss. I did this so your question along with @sueinmn's fabulous response would also help others who are talking about this and keep all the suggestion in one place.

I agree with consulting a nutritionist. Esssbeee, in addition to weight loss, have you loss your appetite? Or is eating not a problem, it's just that the weight doesn't stay on?

@esssbeee Hi there. I too lost weight rapidly at the height of my infection. I went from 142 lbs to 116. I was losing like 4 lbs aweek. I thought surely I must have cancer. Once the antibiotics started kicking in and killed the infection; my weight came back on. I am now at 130 lbs and do not wish to add more. I still eat food with fattening ingredients to maintain my weight. I move a lot so I am not likely to become overweight. I know it is hard to eat when you have no appetite whatsoever and nothing tastes good. You have to be creative and make what food you can eat really count. Rice and avocados are very high in calories. Cucumbers and celery are good inflammation killers. In fact, celery juice is now a big thing for treating inflammation (which is the root of most diseases). Keep me posted on how you are doing and if you come across some good high calorie recipes.