Yeah, indeed font is critical as i did notice too. Verdana works best, but alas as i learned from a math-teacher that needs and uses the codecogs editor in Lectora, Latin Roman is apparently the common choice for equations. The same editor on Codecogs works quite well, changing fonts and all works good ( untill today that is, site down atm )
but inside Lectora Online the editor fails often.
What i did notice however is that reselecting a textfield that has been edited and equations been added often solves it, and then it publishes properly. However it is trial and error. That shouldnot be the case, and indeed also sent it in as a bug.