Korea did not have its own writing system and used Chinese characters until the invention of hangul in 1443. What was the reaction in Ming China to Korea's invention of its own indigenous written language?

This might not directly answer your question but:

What often gets overlooked is that there have been several writing systems used to write Korean, of which the unsuitability of those for their purpose was one of the reasons Hangeul was conceived. Hangeul might not have been perceived as the groundbreaking innovation as it is popularized as in the modern day and had a precedence in its predecessor of Idu, that unlike Hanja was largely the script used by the middle class.

Idu, Hyangchal and Gugyeol are the known writing systems that were used to write down Korean prior to the invention of Hangeul (Gugyeol was a glossing practice similar to the Japanese Kunten (訓点), which also appears to have developed under the influence of Gugyeol for similar reasons).

To note, there were two main readings for Chinese characters being eum (音) - read them with their Sino-Korean pronounciation - and hun (訓) - the character is used for its meaning and represents a vernacular sound value.

Idu, which was the most full fledged one of the three was written in Korean word order, with (Old/Middle) Korean syntax and hun usage of the Hanja characters and made use certain characters for Korean morphemes such as 中, perfective 在, conclusive 之 and instrumental 以. Yet this remained a difficult workaround just to write down what you speak every day anyways and could tell it to someone instead.