yes it seems reason by "load_dotenv" function. I didn't used this module but probably this function pull environment variables from .env file.
I know your codes are running on Desktop successfully. Because your ".env" file location represented in folder. I think this .env file doesn't include on apk like other ttf(type font files). Normally, if you put on building apk folder of your font name file, automatically includes on app this font file without define any path on phone storage. But I don't sure, you have to define path for this ".env" file, or add ".env" on buildozer needed place (.png, .jpg, .ttf...extension).
If you think problem is at it. you can try to check manually. so put .env file mobile phone storage(-Android phone-) and define needed path as "
/sdcard/<createdfilefortest>/.env.env" do not forget extension ".env" and give needed permissions for reach on storage. like this thing,
from android.permissions import request_permissions,
Permission
request_permissions([Permission.READ_EXTERNAL_STORAGE,
Permission.WRITE_EXTERNAL_STORAGE])