OP said the company is taxed as a corporation - then it will be taxed as a Spanish corporation.
If it's treated as transparent in the US, then the other country can still tax it like a corporation. @TheCryptoAnt wrote Spain accepts US LLCs as transparent, so then that wouldn't be a problem in Spain.
But many other countries still see "disregarded entities" as corporations. That's because they have to find the "equivalent" local entity type and in many countries, only corporations award limited liability.
If it's treated as transparent in the US, then the other country can still tax it like a corporation. @TheCryptoAnt wrote Spain accepts US LLCs as transparent, so then that wouldn't be a problem in Spain.
But many other countries still see "disregarded entities" as corporations. That's because they have to find the "equivalent" local entity type and in many countries, only corporations award limited liability.