Math fake strip markers (UNIQ--postMath-…-QINU) appear in the table of contents when a heading contains <math>. For example: https://en.wikipedia.org/wiki/Student%27s_t-distribution#toc
This seems to be a new issue, and likely related to the changes from T287767 / T295187.
There are known issues where these markers appear in search results (T242327) and in the id/href attributes (T94344#4804654), but appearing in the visible content is new, as far as I know.
(Originally reported in T138453#7491782)