Steps to reproduce
I typed in this latex
$$ x \over y $$
Then I typed
$$ x \over y $$
Expected result
Same as above but bigger
Actual result
Environment
0.6.5
I typed in this latex
$$ x \over y $$
Then I typed
Same as above but bigger
0.6.5
This is not a bug.
I am not sure if it is a good idea to put latex in headings.
Anyway, if you want to do that I would not use the block latex syntax but the inline latex syntax
# title $x\over y$