I started editing text content of MathML tags in Web Inspector. Sometimes the rendering would update, sometimes not. When it didn't, resizing the window so the text has to be reflowed made my changes appear.
I think this is related to Bug 47781.
See bug 57695 for a specific example and a test case.
See also bug 57696.
The implementation has changed quite a lot after https://trac.webkit.org/wiki/MathML/Early_2016_Refactoring#Phase1DONE (except for mfenced). I think we can close this bug and open more specific issues. I expect that phase 3 could also improve the rendering update.