Is it normal to keep undereating?

Yes and no. Some people just eat less than others. But if you think your eating WAY under what others eat and your extremely underweight or might become underweight soon, you might need to get professional help. If it’s just a little less than everyone else your fine. As long as you still eat and are not losing weight rapidly. I may also be wrong about all of this info since it’s just my personal opinion. I’m in no way a medical expert.

That is not healthy at all it's actually far healthier to slightly overeat that is to undereat.
the different is being if when you overeat you will gain weight, that you can do go for a walk or jog and burn the calories away if you're under eating you're just starving your body and your just dieing. Seriously you're going to have so many more problems.

I used to be underweight and back then I was always feeling ill and in pain and the doctors said to me there's not much they can do to help. The only thing they could is recommend that I start eating little and often. Slowly build up my appetite and fat. Noe these days I'm overeating daily and slowly gaining weight and fat. And I've been feeling so much better for it. The pain has stopped. And the doctors have said my overall health has improved dramatically. Am a so much happier girl for it. and my physiologist has told me that I'm safe to continue to gain more weight in fat if I so wished. So I Shall :) I am happy healthy now.

I barely eat once a day and I still have trouble losing weight.