Mom is losing weight but eats at least 1 good meal a day. Why?

Could be all sorts of reasons.

If you were to get obsessive about it and count up the total number of calories she is consuming, and compare it to her Base Metabolic Requirement (which you can estimate based on her height, weight, build and level of activity), you may find that although she seems to be eating well - less than a third of her meal? It's good, but it's not what you'd call a hearty appetite, is it? - she isn't in fact meeting her intake requirement.

Or... Her gut might not be absorbing nutrients, her metabolism could have gone awry, or there could be other, new demands on fat stores. And is she drinking as much as she used to? Water is heavy, maybe she's a bit dehydrated? Don't worry about it, just make sure a drink is available if she wants it.

It's probably best to focus on the upside - her enjoyment of whatever she feels like eating and drinking. Don't you think?

I love cooking by myself, and I have a lot of cook books with thousands of recipes. I enjoy trying different recipes and generally enjoy the food, but due to my work and tight schedule I’ve developed an issue with my weight. I was aware that I’m not going to start exercising, so I had to change something in my diet. As I mentioned before, I have a lot of cook books in my collection, but I started to purchase and collect healthy food cook books, so I could lose my weight and retrieve my previous condition. I’ve been cooking according to these cook books since, but I didn’t have any specific results. I lost some weight, but it wasn’t any spectacular. But, I came across one specific book on the internet, and because I collect cook books and recipes, I decided to give it a try. So, I gave it a try and I’ve been cooking according to this book so far, and I was amazed by the results. I’ve lost 6 lbs in only 3 weeks. This was exactly what I was looking for. I didn’t want to lose the weight very quickly, but in a more healthy manner instead. So, I’m quite happy with this book, it’s gonna be part of my cook book collection for a long time and I definitely recommend it to everyone.

What we think of as the "rules" about how many calories it takes to maintain our weight or how many calories less we'd have to eat each day to lose a pound often don't apply to those whose brains are broken. Brains control metabolism as well as almost everything else. I think it usually results in weight loss (it did for my husband) but I've seen posts about weight gain during dementia, too.

Hospice is right.

Keep offering her foods she really enjoys, for the sake of her enjoyment. Don't obsess about nutrition or weight at this point.

According to
Livehealthy.com;
A study published in the “Journal of the American Geriatrics Society” in 2007 examined hospitalized geriatric patients admitted to rehabilitation or acute care units. The researchers found that the average resting energy expenditure of these elderly patients was 8.5 calories per pound of body weight daily. However, researchers report that this energy intake covers minimal nutritional needs, and many hospitalized elderly patients would benefit from higher intakes. Based on these findings, a 115-pound bedridden elderly woman requires a minimum of 978 calories daily.
I don't think two-30% meals (or 60% of one meal) would supply more than 600 calories. Depending on how much Ensure is taken, she wouldn't be close to even the minimum mark.

Unfortunately, this is normal as the body starts preparing to pass away.
Offer frequent snacks in small amounts and favorite foods, even junk food (which usually has a high calorie content).