Same symptoms as in T4144, T7020, and T10970: A <math> tag inside of wikilink text (e. g. `[[Pythagorean theorem|<math>a^2+b^2=c^2</math>]]`) will produce gibberish like

'"

UNIQ--postMath-00000008-QINU"'

However, this only happens in MathML mode. When rendering PNGs or LaTeX source code, it works, which I suppose would be why the above tickets are all closed.