Properly its neither a "principle agent problem" nor a "principal agent problem" but a "principal-agent problem" (that is, the problem is a conflict between the principal and the agent [both nouns] nominally acting on behalf of that principal, not a problem with the principal [adjective] agent [noun].)
That's still two words and only one was wrong. Hyphenation is not mandatory, there is no formal description of a "problem with the principal [adjective] agent [noun]" to be disambiguated from.
