mathpix-markdown-it did not render LaTeX theorem environments like \begin{theorem}...\end{theorem}, \begin{lemma} or \begin{proof}. Users coming from the Mathpix OCR API output expected theorem and theorem-like environments to typeset like they do in LaTeX documents. After several users asked for it over two years, the maintainer announced theorem support with docs at doc/theorems.md in the repo.

Theorem support was added to mathpix-markdown-it; the maintainer announced it on the issue and pointed to the docs at doc/theorems.md in the repo. Update to a release that includes theorem support and use the documented syntax for theorem, lemma, proof and other theorem-like environments. Source: https://github.com/Mathpix/mathpix-markdown-it/issues/77

Source: https://github.com/Mathpix/mathpix-markdown-it/issues/77