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.
unusual but nor impossible. for example, say the trust held non-dividend paying stocks and sold some of these. the gain would be part or corpus (principal of the trust). unless directed by the trust to distribute the gains (or at the discretion of the trustee if permitted, the trust would pay any income taxes and thus distributions would be tax-free to you. we know nothing about the trust's assets or its provisions. in you are concerned contact the trustee for an explanation.