Betta Fish: Very confused as to what’s going on in this tank

Here’s the story: two weeks ago I got a small female koi betta. She was very pale and clearly pre-glow up. I figured she’d grow into herself so I brought her home. BUT- I didn’t have a tank set up for her yet. I know, putting a fish into an uncycled tank is a stupid idea. Honestly boo me, I deserve it. I’m not proud of it, but I thought if I monitored the tank closely it would work out. To sort of justify my decision making she was in a divided tank at my LFS, and they literally told me to “get her out of here” and explained their town water is too hard and the bettas don’t thrive in it. My town water is very soft and low in pH, which is why I keep South American fish and labyrinth fish. So I gathered my old fish supplies I had in storage and set up a planted 5 gallon tank. I introduced her a few hours after. I have tried to feed her multiple times a day, she always chomps her food, but then spits it out. Every. Time. I’ve tried hikari micro pellets, fluval bug bites: betta formula, and frozen bloodworms. She’s always interested but never keeps down the food. It’s been two weeks and she’s still incredibly inquisitive and active, which is crazy because I’ve never seen her not spit out her food. So I’ve been monitoring the tank for two weeks and testing the water with my mostly reliable test strips. The water quality always came out looking just fine, with almost zero nitrates and zero ammonia or nitrites. So this morning I pull out the API master test kit expecting to see SOMETHING wrong. With all the food my betta rejects, surely it’s fouling up the water regardless of what the strips say. But there’s zero ammonia and nitrates? My betta still won’t keep down food, the tank seems stable, and she’s still active despite not eating. Does anyone know why everything is going so wrong but so right?