Couple of notes:
Why would ACF block math in one type
I didn't understand your description the first time. This is not ACF.
According to the specs the <math> element is considered valid phrased content
It actually falls under 3 categories http://www.whatwg.org/specs/web-apps/current-work/multipage/the-map-element.html#mathml but as you have already mentioned MathML is not used with this plugin so let's leave this part.
when I use the inline editor on phrased content like a <p>.
This is flow element.
Now the problem.
Paragraph is flow element that can only have phrasing elements inside it. When you initialize inline editor on Paragraph element, plugins like Format Dropdown, Table or Blockquote are completely removed so that tags like Div or H1 weren't inserted into Paragraph (not allowed in HTML).
Mathematics plugin inserts images and spans so it should work but for some reason it is disabled. This plugins isn't removed but it may be missing some code to make it enabled in elements like Paragraphs (just me guessing).
