It's easy to find studies connecting metformin to weight loss and loss of appetite. I'll leave that to you, just use Google and you'll be swamped with evidence.

You say you've never heard of metformin affecting food palatability and taste? Did you search the web at all? Just use Google if you want anecdotal examples. For example:

https://forum.diabetes.org.uk/boards/threads/loss-of-appetit...

I joined as after 18 months on metformin, my appetite and enjoyment of food is constantly declining. I’ve lost about 10 kg over that time, which is not a problem. But I’ve lost my appetite and, as a keen cook and someone who has always enjoyed good food, I no longer look forward to meals and get little pleasure from food.

or

https://www.diabetes.co.uk/forum/threads/anyone-experience-l...

Metformin changed the taste of food slowly during the time I took it. It finally got to the point where all vegetables tasted like burnt plastic. This meant that I didn't want to eat them.

And so on. Now you've heard of it! All it took was a web search.