I have had trusts pay me, but always the income. I have never heard of a trust that pays principal (corpus) only. I have only received corpus amounts when a trust ended.
You write, no DNI. Is this trust invested in such a way that it receives no interest, dividends or other forms of income?
You may be receiving a K-1 to show that the amounts you received are principal and not income. Is the trust reporting the gain to you for taxation or is the trust paying the taxes on the gain before distribution.