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listed several possible reasons

You don't have imports, only production. In the case of a closed economy like the USSR, this is normal, but you are trading. Your currency strengthens, prices for the goods you produce rise, and their competitiveness, despite the large quantity produced, decreases (and there's also an embargo). Isn't that right? In the end, the market balances everything out, but not in your favor.
In reality, it is considered dangerous to have too large a trade surplus; in some cases, it can even be worse than a deficit...