Welcome : ) And to the Forum as well!
You might want to consider upping her grain a bit if only feeding once a day and as you say, she is lean.. Not unusual at all for a TB to need more than one daily feed. I don’t know what brand of Senior you feed but check manure for undigested grains ( often a sign of being in need of a float). If she is hungry, this can inspire nibbling… There could also be some type of ulceric activity which is also common among many performance horses, particularly OTTBs. There are several ways you can go to address this but finding the actual cause, if possible, can save time, cash and health. Also note: any dietary changes, up, down, added or subtracted, can take even up to a couple months to show any result.
Chat with your vet. Always the best way to go. And seriously, good luck : )
Sure there's right and wrong but mostly there's just a whole lotta different.