Could she be cold? Try a blanket? As much as I am not a blanket fan, they have their place. Sometimes, the weight loss is simply from generating heat and a blanket will definitely help. My guys, the ones that I cover, will start to loose weight in the fall when the temps get consistently below 30F if I don’t cover them with something. I have a few elders (3 of 6 are over 25) in my yard as well and as much as they CAN look right, it is a totally different ballgame to maintain them. Elders do not absorb anything dietary as well as the younger ones, harder to get the nutrients into their system and they get colder easier.
I tend to up the grain a bit (Triple Crown Senior here) early fall, then after the first real frost and if weight is up, I back off the grain and feed free choice hay for the entire winter. Grain is still fed but back to summer amounts (the difference being less than a 1/2 lb per horse). IF weight is still lost or not gained after an increase in grain, THAT horse gets covered.
With the beet pulp, I am sure your girl is getting a good amount of water but there really is no such thing as too much in winter. A little more water, hot if possible, plenty of hay and BP, maybe reconsider your grain, look for a higher fat, lower NSC, lower sugar content to give her a chance to absorb them before she spends them. Note the status of her teeth, too.
Nothing was mentioned in your post but I will offer this anyway: Be sure she is physically comfortable. Pain/stress can drop weight faster than anything.
The elders can certainly put grey hair on your head but they CAN rally and maintain. You might have to try different combinations and different components to get her needs met.
Good luck to both of you and don’t forget: She didn’t make it to her 30’s by being poorly kept. GOOD on YOU!!
Sure there's right and wrong but mostly there's just a whole lotta different.