If you cannot get a satisfactory answer from Amtrak, and your situation is serious enough (sounds like it is), I wouldn't chance it.
You can prepare your own food or bring prepared food you are sure about ingredients for and not worry about it. I find home-dehydrating solves a lot of these issues. Just rehydrate on board. But that's a lot of work, and requires a home dehydrator and other items you may not have. I used to do that a lot.
These days, here is my go-to: https://www.packitgourmet.com/
They primarily serve the backpacking market and have an excellent reputation, but many others have discovered them and love PackIt too! The pandemic has brought them new customers and positive reviews.
PackIt Gourmet sells dehydrated and freeze-dried ingredients for DIY. But they are best known for a complete curated menu of meal pouches for all three daily meals that you just add hot or cold water to, let rehydrate, and eat with a long spoon or spork right out of the pouch. They also have desserts and snacks. Their website is a treasure trove of info. Ingredients for each meal can be found on the packaging images on the site, and their excellent customer service folks can answer any questions not answered there, like your rather specific issue.
I don't mean to sound like a shill for this one company. I have no financial interest in it. There are other companies serving the same market with comparable products, so let Google be your friend and compare if you have time. This is just the one I know (well).
Having eaten railroad fare for over 40 years, I will admit nothing like this will satisfy pax nostalgic about the good old days. But IMHO it's better than much of what Amtrak is serving these days, and less expensive. It's a shame sleeper pax can't simply pay to travel and opt-in or opt-out of having meals included.
Best wishes...