I think the problem is related to webpack, I had the same problem on a next.js project and here is how I did to fix the problem.
I created a next.conf.js file in my root folder where there is my .env file, I imported dotenv and I exported the module where there is all my environment and these variables as if below.
Finally in my index file and my components I don’t need to import dotenv, I just need to paste my process.env.API_URL.
I hope this example solves your problem.